Smith v. Maryland
The decision holding that use of a pen register to record numbers dialed from a telephone was not a Fourth Amendment search.
Executive Summary
Police investigating a robbery asked the telephone company to install a pen register at its central offices to record the numbers dialed from Michael Lee Smith’s home telephone. No warrant was obtained. The device did not capture the contents of calls; it recorded addressing information. The resulting data helped connect Smith to the victim, and additional evidence was later obtained under a search warrant.
The Supreme Court held that installing and using the pen register was not a Fourth Amendment search. The majority reasoned that Smith had no legitimate expectation of privacy in numbers he voluntarily conveyed to the telephone company for routing and billing. The Court also observed that pen registers had limited capabilities and did not reveal conversation content.
The decision established a broad proposition: information knowingly exposed to a third party may fall outside the Fourth Amendment’s privacy protection. Yet the decision must now be read with Carpenter v. United States, which declined to apply that proposition automatically to seven days of historical cell-site location information. For agencies, the lesson is not that all non-content metadata is warrantless; the quantity, precision, duration, automatic generation, and revealing nature of modern data can alter the constitutional analysis.
Historical Context
After Katz, courts asked whether a person had a legitimate expectation of privacy in particular information. United States v. Miller had held that bank records were business records exposed to bank employees. Smith extended similar reasoning to dialed telephone numbers and became a foundation of the third-party doctrine.
The technological setting was comparatively modest. A pen register recorded numbers dialed from a single landline for a short period. It did not identify the speaker, record words, reveal a cellphone’s location, or aggregate a person’s movements across time. Those limitations matter when translating the decision to modern systems.
Facts
Patricia McDonough was robbed in Baltimore. Afterward, she received threatening and obscene telephone calls from a man who said he was the robber. Police traced one call and developed Smith as a suspect. Officers observed Smith’s vehicle and obtained identifying information.
At police request, the telephone company installed a pen register at its central office. The register recorded that a call was placed from Smith’s home to McDonough’s number. Police then obtained a warrant to search Smith’s home, where they found a telephone directory with a page turned down at McDonough’s name and other evidence. McDonough identified Smith in a lineup.
Procedural History
Smith moved to suppress all evidence derived from the pen register, arguing that its warrantless use violated the Fourth Amendment. The Maryland trial court denied the motion, and Smith was convicted. The Maryland Court of Appeals affirmed. The Supreme Court granted certiorari to decide whether the pen register’s installation and use constituted a search.
Constitutional Question
Does the government conduct a Fourth Amendment search when, without a warrant, it uses a pen register installed at a telephone company’s facilities to record numbers dialed from a suspect’s telephone?
Holding
No. The Court held that Smith had no legitimate expectation of privacy in the numbers he dialed because he knowingly conveyed them to the telephone company, and because the pen register did not acquire the contents of communications. No warrant was constitutionally required on the facts presented.
Court’s Reasoning
The majority applied the privacy framework associated with Justice Harlan’s Katz concurrence. It first doubted that Smith actually expected dialed numbers to remain private. Telephone users understood that numbers had to be transmitted to the company and that companies could use them for legitimate business purposes.
The Court then concluded that any subjective expectation was not one society was prepared to recognize as reasonable. By using the phone, Smith voluntarily conveyed numerical information to the company and assumed the risk that the company would reveal it to police. The Court analogized to business records disclosed to banks in Miller.
The majority stressed the narrow technical capacity of the device. A pen register disclosed only numbers dialed, not the substance of a conversation. That content–non-content distinction later became embedded in federal electronic-communications statutes, though statutory rules can require process even when the Constitution does not.
Modern readers should avoid converting the majority’s “voluntary conveyance” language into an unlimited rule. Digital services now generate information continuously, often as an unavoidable incident of participation in society. Carpenter treated the automatic, comprehensive, and revealing character of CSLI as reasons not to extend Smith mechanically.
Separate Opinions
Justice Stewart dissented, emphasizing that telephone numbers can reveal intimate associations and that numbers dialed from a private telephone were not meaningfully public. Justice Marshall also dissented, questioning the assumption-of-risk rationale and arguing that people should not have to choose between using a telephone and surrendering constitutional protection.
Those dissents anticipated contemporary concerns about metadata. Even without communication content, patterns of calls, contacts, timing, frequency, and duration may reveal political, medical, religious, professional, or personal relationships.
Continuing Significance
Smith remains controlling for traditional pen-register facts, but its reach is contested when technologies collect richer or more persistent data. Courts assessing tower dumps, geofence returns, internet metadata, smart-device records, and platform logs increasingly consider not just whether a provider holds the data, but what the data reveals and how it was generated.
For agencies, constitutional sufficiency and statutory compliance are separate inquiries. The Pen Register Act, Stored Communications Act, state constitutions, and state statutes may demand court orders or warrants beyond the federal constitutional floor. Sound practice is to identify the precise data field requested, retention period, technical capability, and governing process before collection.
What Smith Does Not Hold
Smith does not hold that all information possessed by a service provider is categorically unprotected. It does not govern communication content. It does not decide the constitutionality of long-term location tracking, bulk collection, or modern datasets that reveal a detailed portrait of life. It also does not displace statutes or state constitutional provisions that provide greater protection.
Shield Practice Notes
Investigators
Identify whether the request seeks addressing data, content, location, or a combination. Do not label all provider data “metadata” and assume the same process applies.
Warrant drafting
Describe each requested data category, the relevant account or device, the time window, and the investigative nexus. Use a warrant when modern data is precise, persistent, or revealing.
Policy and training
Teach the difference between the federal constitutional floor and statutory or state-law requirements. Build current legal-process charts for common providers.
Discovery
Preserve legal process, provider returns, filtering steps, and analyst notes so the collection and interpretation of data can be reconstructed.
Key Quotations
Primary References
- Smith v. Maryland, 442 U.S. 735 (1979).
- Carpenter v. United States, 585 U.S. 296 (2018).
- 18 U.S.C. §§ 3121–3127.