To investigate the utility of testing all tuberculosis patients for HIV infection in Los Angeles, we prospectively evaluated 183 hospitalized patients with acid-fast smear-positive tuberculosis who gave no history of HIV infection. HIV serologic testing was performed, and the presence of risk factors for HIV infection was assessed by interview and review of the medical record. Thirty-three patients (18%) were infected with HIV. Based on data obtained by interview, 124 (68%) of 183 patients had at least one risk factor for HIV infection. The most common risk factors were related to heterosexual transmission of HIV (prostitute contact, multiple sex partners, and a history of sexually transmitted disease). These risk factors were frequently not documented in the medical record. Among the 33 patients who were infected with HIV, 30 were identified by interviewing as having significant risk factors. We concluded that most tuberculosis patients in Los Angeles have risk factors for HIV infection and that systematic questioning does not identify risk factors in all HIV-infected tuberculosis patients. These data support current recommendations to screen all tuberculosis patients for HIV infection.