This is an audio case brief of Smith v. Maryland, 442 U.S.735 (1979). The audio brief provides a full case analysis. However a written summary of the case is provided below.

Table of Contents


On March 5, 1976, Patricia McDonough was robbed in Baltimore, Md., She gave the police a description of the robber. She also described a 1975 Monte Carlo automobile that she had observed near the scene of the crime. After the robbery, McDonough began receiving threatening and obscene phone calls from a man identifying himself as the robber. On one occasion, the caller asked that she step out on her front porch. She did so, and saw the 1975 Monte Carlo she had earlier described to police moving slowly past her home.

On March 16, police spotted a man who met McDonough’s description driving a 1975 Monte Carlo in her neighborhood. By tracing the license plate number, police learned that the car was registered in the name of Michael Lee Smith.

The next day,  at the request of the police, the telephone company installed a pen register at its central offices to record the numbers dialed from the telephone at Smith’s home.

The police did not get a warrant or court order before having the pen register installed. The register revealed that on March 17 a call was placed from Smith’s home to McDonough’s phone. On the basis of this and other evidence, the police obtained a warrant to search Smith’s residence. The search revealed that a page in Smith’s phone book was turned down to the name and number of Patricia McDonough. The phone book was seized. Smith was arrested, and a six-man lineup was held on March 19. McDonough identified Smith as the man who had robbed her.

Smith was indicted in the Criminal Court of Baltimore for robbery. By pretrial motion, he sought to suppress “all fruits derived from the pen register” on the ground that the police had failed to secure a warrant prior to its installation. The trial court denied the suppression motion.

The Court of Appeals affirmed the judgment of conviction.

The U.S. Supreme court agreed to review the case to resolve the matter.


Whether the installation and use of a pen register constitutes a “search” within the meaning of the Fourth Amendment.


There is no constitutionally protected reasonable expectation of privacy in the numbers dialed into a telephone system. A person has no legitimate expectation of privacy in information he voluntarily turns over to third parties.


The court begun its analysis by outlining the reasonable expectation of privacy test established under Katz. The Katz reasonable expectations of privacy requires that an individual, by his conduct, has ‘exhibited an actual (subjective) expectation of privacy, and  the individual’s subjective expectation of privacy is one that society is prepared to recognize as reasonable.

In applying the Katz analysis to this case, the court stated that it is important to begin by specifying precisely the nature of the state activity that is challenged. The activity here took the form of installing and using a pen register. Since the pen register was installed on telephone company property at the telephone company’s central offices, Smith obviously cannot claim that his “property” was invaded or that police intruded into a “constitutionally protected area.” Smith’s claim, rather, is that, notwithstanding the absence of a trespass, the State, as did the Government in Katz, infringed a “legitimate expectation of privacy” that he held. Yet a pen register differs significantly from the listening device employed in Katz, for pen registers do not acquire the contents of communications.

Given a pen register’s limited capabilities, therefore, Smith’s argument that its installation and use constituted a “search” necessarily rests upon a claim that he had a “legitimate expectation of privacy” regarding the numbers he dialed on his phone.

But the court found that his expectation of privacy is not legitimate.

And here is why:

Firstly, the court stated, it is doubtful that people in general entertain any actual expectation of privacy in the numbers they dial. All telephone users realize that they must “convey” phone numbers to the telephone company, since it is through telephone company switching equipment that their calls are completed. All subscribers realize, moreover, that the phone company has facilities for making permanent records of the numbers they dial, for they see a list of their long-distance (toll) calls on their monthly bills. In fact, pen registers and similar devices are routinely used by telephone companies “for the purposes of checking billing operations, detecting fraud and preventing violations of law. Additionally, electronic equipment is used not only to keep billing records of toll calls, but also “to keep a record of all calls dialed from a telephone which is subject to a special rate structure.

Smith however counter argued that whatever the expectations of telephone users in general, he demonstrated an expectation of privacy by his own conduct here, since he “used the telephone in his house to the exclusion of all others.

But the court disagreed with him

The court reasoned that the site of the call is immaterial for purposes of analysis in this case. Although Smith’s conduct may have been calculated to keep the contents of his conversation private, his conduct was not and could not have been calculated to preserve the privacy of the number he dialed. Regardless of his location, Smith had to convey that number to the telephone company in precisely the same way if he wished to complete his call. The fact that he dialed the number on his home phone rather than on some other phone could make no conceivable difference, nor could any subscriber rationally think that it would.

Secondly, even if Smith did harbor some subjective expectation that the phone numbers he dialed would remain private, this expectation is not “one that society is prepared to recognize as ‘reasonable. This Court consistently has held that a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties.


The court concluded that Smith can claim no legitimate expectation of privacy here. When he used his phone, he voluntarily conveyed numerical information to the telephone company and “exposed” that information to its equipment in the ordinary course of business. In so doing, Smith assumed the risk that the company would reveal to police the numbers he dialed.

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