To determine the risk of active tuberculosis associated with human immunodeficiency virus (HIV) infection, we prospectively studied 520 intravenous drug users enrolled in a methadone-maintenance program. Tuberculin skin testing and testing for HIV antibody were performed in all subjects. Forty-nine of 217 HIV-seropositive subjects (23 percent) and 62 of 303 HIV-seronegative subjects (20 percent) had a positive response to skin testing with purified protein derivative (PPD) tuberculin before entry into the study. The rates of conversion from a negative to a positive PPD test were similar for seropositive subjects (15 of 131; 11 percent) and seronegative subjects (26 of 202; 13 percent) who were retested during the follow-up period (mean, 22 months). Active tuberculosis developed in eight of the HIV-seropositive subjects (4 percent) and none of the seronegative subjects during the study period (P<0.002). Seven of the eight cases of tuberculosis occurred in HIV-seropositive subjects with a prior positive PPD test (7.9 cases per 100 person-years, as compared with 0.3 case per 100 person-years among seropositive subjects without a prior positive PPD test; rate ratio, 24.0; P<0.0001). We conclude that, although the prevalence and incidence of tuberculous infection were similar for both HIV-seropositive and HIV-seronegative intravenous drug users, the risk of active tuberculosis was elevated only for seropositive subjects. These data also suggest that in HIV-infected persons tuberculosis most often results from the reactivation of latent tuberculous infection; our results lend support to recommendations for the aggressive use of chemoprophylaxis against tuberculosis in patients with HIV infection and a positive PPD test. (N Engl J Med 1989; 320:545–50.)