In Nelson v. United States (1967), the Supreme Court ruled in favor of a defendant who had been convicted for selling obscene material through interstate commerce. The court found that the jury instructions given during Nelson's trial were incorrect because they did not adequately explain to jurors what constitutes obscenity under federal law. Specifically, the judge failed to instruct jurors that they needed to consider whether "the dominant theme of the material taken as a whole appeals to prurient interest." Instead, he told them only that materials could be considered obscene if parts of it met this standard - an interpretation which was too broad and thus unconstitutional according to previous Supreme Court rulings on obscenity laws. Therefore, due to these faulty instructions, Nelson's conviction was overturned.

In the dissenting opinion for Nelson v. United States, Justice Fortas argued that the majority's decision to uphold a conviction based on evidence obtained through an unauthorized wiretap was a violation of Fourth Amendment rights against unreasonable searches and seizures. He contended that allowing such evidence would encourage law enforcement agencies to disregard constitutional protections in their investigations. Furthermore, he disagreed with the majority's view that Congress had implicitly approved warrantless wiretapping in national security cases by not explicitly prohibiting it when they passed Title III of the Omnibus Crime Control Act. Instead, he believed this silence reflected Congressional uncertainty about how to balance privacy rights with national security concerns rather than approval of unrestricted surveillance powers for federal agents.