The aim of this study was to determine the frequency and aetiology of fever of unknown origin (FUO) in patients infected with the human immunodeficiency virus (HIV), to assess the value of the tests used in its diagnosis, and to evaluate possible models of diagnosis for the causes found most frequently. One hundred twenty-eight (3.5%) of 3603 hospitalised HIV-positive patients evaluated from October 1992 to December 1993 had FUO, defined by established criteria. Eighty-six percent of patients with FUO had previously progressed to AIDS. The median CD4+ cell count was 46/mm3. A definite diagnosis was made in 96 (75%) of the 128 patients and a possible diagnosis in 24 (18.7%). whilst no diagnosis was made in eight cases (6.2%). Tuberculosis (48.3%), visceral leishmaniasis (16%), and infection by Mycobacterium avium complex (6.9%) were the diseases found most frequently. The most useful diagnostic tests were liver biopsy (68.9%) and bone marrow aspirate/biopsy (39.7%). It is not possible to predict clinically the cases of FUO due to tuberculosis, whilst thrombocytopaenia < 100,000 cells/mm3 alone is useful for differentiating the cases of visceral leishmaniasis, with a negative predictive value of 95.2%.