PIP: The authors present the case of a 60-year old woman who developed acquired immunedeficiency syndrome (AIDS) 43 months after a blood transfusion. The blood donor was identified as a sexually active homosexual man who frequently used intravenous drugs. Although he was asymptomatic, physical examination revealed generalized lymphadenopathy of cervical, axillary, and inguinal areas. 47 months after his blood donation, the donor underwent lymph node biopsy which revealed follicular hyperplasia. He was seropositive for antibodies to human T-lymphotropic virus type III (HTLV-III) and had serologic evidence of prior infection with cytomegalovirus, Epstein-Barr virus, and hepatitis B virus. On the other hand, he had a normal total lymphocyte count and normal T-cell subsets. Striking was the difference between the seropositivity of the high-risk donor and the seronegativity of the patient with transfusion-associated AIDS with respect to cytomegalovirus, Epstein-Barr virus, and hepatitis B virus. The demonstration of apparent transmission of HTLV-III by blood transfusion and the inability to detect evidence of infection with other viruses strongly indicate a primary etiologic role for this virus in the pathogenesis of AIDS. The high-risk blood donor continues to be an asymptomatic carrier of HTLV-III. The authors are currently comparing the HTLV-III isolates obtained from the blood donor with the HTLV-III-related RNA sequences detected in the spleen of the patient in order to determine whether molecular changes occurred after transmission of the virus that might be related to the development of AIDS.