The purpose of this study was to determine the prevalence of fever of unknown origin (FUO) in a cohort of HIV positive patients and to describe their evolution and the final diagnosis. The clinical records of 412 patients followed from January 1987 to December 1990 at our HIV outpatient clinic were reviewed: in 151 patients 255 episodes of fever had been observed of which 22 (in 21 patients) met the criteria for FUO. 19 patients (90%) presented with a CDC/WHO stage IV HIV infection and the mean CD4+ lymphocyte count was 0.160 G/l. The etiology was ultimately determined in 13/22 episodes (3 Pneumocystis carinii pneumonia, 3 invasive infections due to atypical mycobacteria, 2 bacterial pneumonia, 1 Cytomegalovirus colitis, 1 Isospora belli enteritis, 1 visceral leishmania, 1 candida septicemia and 1 lymphoma). In 6/22 episodes, the fever subsided after zidovudine was started and was therefore attributed to HIV itself. In 3/22 episodes no etiology was found. In conclusion, this series shows that FUO is usually seen in advanced HIV infection and that it often represents an early sign of opportunistic infection. This observation underlines the importance of follow-up, since it finally served to detect the etiology of FUO in 86% of cases. Trial treatment with zidovudine can be useful where no pathology has been discovered despite 3 weeks' follow-up and appropriate investigations.