To test the hypothesis that HIV infection can modify the clinical characteristics of tuberculosis, 65 consecutive cases of tuberculosis in HIV-seropositive patients diagnosed in Barcelona (Spain) were compared with 65 HIV-seronegative controls matched for age and sex. Thirty of the 65 cases were accepted as AIDS cases (August 1987 Centers for Disease Control criteria) only because of the tuberculosis. Among the cases 54 (83%) were parenteral drug addicts and 88% were males. The tuberculosis was pulmonary or pleural in 62 controls (96%) but in only 25 cases (39%; P < 0.0001). Lymph nodes were involved in 25 cases (39%) and in none of the controls (P < 0.0001). Disseminated forms of tuberculosis were present in seven cases (11 %) and in no controls (P < 0.007). Bone, joints and central nervous system involvement were also significantly (P < 0.05) more frequent in cases. The treatment (isoniazid and rifampin for 6 months plus ethambutol and pyrazinamide during the first 2 months) was always effective. One relapse was detected after a median follow-up of 55 months in cases and none in controls after a median follow-up of 43 months. Twenty-five cases (39%) and 14 controls (22%) developed mild or severe side effects related to the treatment (P < 0.004). In conclusion, most of the HIV-infected patients with tuberculosis were drug addicts with extrapulmonary or disseminated forms. A short course of treatment (6 or 9 months) may be enough but side effects were frequent.