The authors report the natural history of HIV infection in a patient with severe hemophilia A who became HIV-seropositive in 1983 and, four years later, developed full-blown AIDS associated with a disseminated Kaposi's sarcoma. Neutralizing antibody titers against HIV were shown to be inversely disease-associated, while the progression of clinical symptoms was directly related to the decline of T4 cells and the increase of urinary neopterin levels. It is suggested that the presence of an HLA DR 5 phenotype and repeated CMV infection could have been crucial for the development of KS.