In Ybarra v. Illinois, the U.S. Supreme Court ruled that a search warrant for premises does not automatically justify the search of individuals found on those premises. The case arose when police in Aurora, Illinois executed a search warrant at a tavern where they suspected drugs were being sold and searched all patrons present during their operation, including Gregorio Ybarra who was subsequently charged with possession of heroin. In its decision, the court held that "a person's mere propinquity to others independently suspected of criminal activity does not, without more, give rise to probable cause to search that person." It further stated that each patron’s privacy could only be invaded upon credible evidence or reasonable suspicion against them individually rather than as part of an undifferentiated group within the location under investigation.

In the dissenting opinion for Ybarra v. Illinois, Justice Rehnquist argued that the majority's decision was inconsistent with previous rulings and unduly restricted law enforcement's ability to conduct searches in situations where there is reasonable cause. He contended that when a warrant has been issued to search a premises, it should be assumed that all individuals present could potentially conceal or destroy evidence related to the investigation. Therefore, he believed it was unreasonable not to allow police officers to frisk those on site for their own safety and preservation of potential evidence. In his view, this did not violate Fourth Amendment rights against unreasonable searches as long as there were valid reasons for issuing the original search warrant.