The acquired immunodeficiency syndrome was first recognized as a clinical entity in the United States in the early 1980s; however, the issue of when human immunodeficiency virus, the causative agent of the acquired immunodeficiency syndrome, was introduced into at-risk populations in the United States is unresolved. Previously, we reported the case study of a 15-year-old black male who was admitted to St Louis City Hospital in 1968 for extensive lymphedema of the genitalia and lower extremities. Chlamydial organisms were widely disseminated and isolated from numerous body fluids and organs. Over a 16-month clinical course his condition progressively deteriorated, and at autopsy there was widespread Kaposi's sarcoma of the aggressive, disseminated type. Recently performed Western blot and antigen capture assays on serum and autopsy tissue specimens frozen since 1969 have disclosed that this sexually active teenager was infected with a virus closely related or identical to human immunodeficiency virus type 1. The clinical and immunologic findings together suggest that an immunosuppressive retrovirus existed in the United States before the late 1970s.