Drug interactions between rifamycins and highly active antiretroviral therapy (HAART) have raised concerns in the treatment of human immunodeficiency virus (HIV)-infected patients with tuberculosis. We conducted a study of this interaction by measuring serum drug levels of all HIV-infected patients with tuberculosis who were admitted to A. G. Holley State Tuberculosis Hospital (Florida) from October 1997 through December 1998, who were concomitantly treated with rifabutin and HAART. All 25 patients studied became culture-negative within 2 months of initiation of therapy for tuberculosis and remained negative for a median of 13 months follow-up after completion of therapy. HIV viral loads (mean+/-SEM) decreased significantly from 4.95+/-0.21 log10 copies/mL before initiation of HAART to 2.77+/-0.07 log10 copies/mL before discharge (P<.001); 20 of 25 patients achieved viral loads of <500copies/mL. In summary, the concomitant use of rifabutin and HAART can lead to successful treatment of HIV-infected patients with tuberculosis without increased side effects.