In the case of Mathis v. United States, 1967, the U.S. Supreme Court ruled on a matter concerning Fifth Amendment rights and self-incrimination during tax investigations. The appellant, Jack Mathis was convicted for willfully attempting to evade income taxes by filing fraudulent returns. During his trial, evidence from an Internal Revenue Service (IRS) investigation was used against him which he claimed violated his Fifth Amendment right against self-incrimination as he had not been warned about potential criminal liability before being questioned by IRS agents. The Supreme Court held that although warnings are necessary when a taxpayer is in custody or under compulsion to speak due to other circumstances such as subpoenaed documents or testimony; they were not required in this instance because Mathis voluntarily provided information without any form of coercion involved. Furthermore, it was noted that even if there had been some degree of compulsion present during the initial stages of inquiry by IRS agents; once formal charges were filed and proceedings began - all subsequent statements made would be considered voluntary unless proven otherwise.

In the dissenting opinion for Mathis v. United States, Justice Harlan argued that the majority's decision was inconsistent with prior case law and overly broad in its interpretation of "custodial interrogation." He contended that a person should not be considered in custody merely because they are confined to a hospital bed or otherwise physically restrained. Instead, he believed that custodial status should depend on whether an individual is subject to formal arrest or restraint on freedom of movement at the level associated with formal arrest. Furthermore, he disagreed with the majority's view that Miranda warnings were necessary before any questioning by police officers could occur while someone was hospitalized under guard. He felt this would unduly limit legitimate investigative techniques and potentially hinder crime-solving efforts.