The case records of consecutive patients admitted to a specialist HIV/AIDS inpatient unit between 1989 and 1993 with pyrexia of undetermined origin (PUO) were reviewed in order to determine the eventual diagnosis. Seventy-nine episodes occurred in 75 patients; 52 patients had a prior AIDS defining diagnosis. CD4+ lymphocyte counts ranged widely, 0-0.79 (median=0.04) x 109/1. Infections were found in 63 episodes (79%), including mycobacterial infection in 41 episodes (53%) and bacterial infection in 12 episodes (15%). Tumours were found in 6 episodes (8%), 5 of these were lymphoma. Factitious fever accounted for 2 episodes (3%) and connective-tissue disease for 1 episode (1%); no definite diagnosis was reached in 7 episodes (9%). PUO in HIV positive patients is commonly due to infection or tumour. Unexplained fever in this patient group should not be ascribed to HIV infection itself and should be vigorously investigated to find a cause.