During his forty-eight years as director of the Federal Bureau of Investigation, J. Edgar Hoover scrupulously maintained secret office files and arranged for special filing procedures to safeguard "sensitive" information. None of the documents in five separate files was intended ever to be disclosed. Athan Theoharis, one of the FBI's most diligent critics, secured these materials after intensive research in FBI files using the Freedom of Information Act. Published here with supporting documents and Mr. Theoharis's authoritative comments, they reveal the shocking extent of FBI activities in collecting and using defamatory information about prominent Americans and political groups.