The records of the San Francisco General Hospital (SFGH) Blood Bank were reviewed, and 263 likely AIDS and AIDS-related complex (ARC) patients were identified, who received 1545 units of packed red cells (PBRCs) between July 1, 1987, and June 30, 1988. A probability sample of 80 of these patients was selected randomly for detailed chart review. Of this sample, 78 (98%) were confirmed to have AIDS (86%) or ARC (14%). On the basis of the yearly census of the SFGH AIDS clinic, a transfusion incidence of 0.89 PRBC units per patient per year for patients with AIDS and 0.27 PRBC units per patient per year for those with ARC was estimated. Whereas 26 percent of the 177 transfusions studied in detail involved more than one associated (possibly causative) factor, antimicrobial drug therapy, zidovudine therapy, and disseminated Mycobacterium avium complex (MAC) infection were the sole associated factors in 20, 14, and 12 percent of the transfusions, respectively. To assess the role of MAC, the 263 transfused patients were compared with the 574 patients whose blood was submitted to the SFGH Mycobacteriology Laboratory during the same period. Patients whose blood yielded MAC had a relative risk of 5.2 for transfusion-requiring anemia. In 80 percent of cases, the patient returned home after transfusion. Most PRBC transfusions administered to AIDS or ARC patients were optimal therapy.