In the case of Procunier v. Martinez, 1973, the U.S Supreme Court ruled that prison officials could not censor inmate mail unless it was necessary to do so for security reasons. The court held that censorship was only justified if it furthered one or more substantial governmental interests related to prison security and order. This decision came after a group of California prisoners and their correspondents filed a lawsuit challenging regulations allowing prison authorities to read and censor outgoing mail from inmates. They argued these practices violated their First Amendment rights to free speech as well as their Fourteenth Amendment due process rights because they were not given an opportunity to challenge the censorship decisions. The Supreme Court agreed with them in part, ruling that while prisons may have legitimate reasons for controlling some aspects of inmate correspondence, any restrictions must be narrowly tailored and cannot go beyond what is necessary for maintaining order and discipline.

In the dissenting opinion for Procunier v. Martinez, Justice Rehnquist argued that the majority's decision to grant prisoners unrestricted mail privileges was a mistake. He believed it would lead to an increase in prison violence and unrest by allowing inmates to freely communicate with outside criminal elements or coordinate disruptive activities within prisons. Furthermore, he contended that this ruling undermined prison officials' ability to maintain order and discipline among inmates effectively. Rehnquist also disagreed with the majority's view on censorship of prisoner correspondence, arguing that such measures were necessary for maintaining security rather than violating First Amendment rights as claimed by the majority. He asserted that courts should defer more heavily towards decisions made by prison administrators who have firsthand experience dealing with these issues.