We evaluated the presenting characteristics, response to therapy, and outcome for 46 patients infected with Mycobacterium kansasii and human immunodeficiency virus (HIV). M. kansasii infection occurred late in HIV disease (mean CD4 lymphocyte count, 52.4/mm3), when most patients had already developed AIDS; 91.3% of the patients had pulmonary involvement, and 21.7% had disseminated disease. Clinical and radiographic findings were consistent with pulmonary disease and had been present for approximately 4 weeks. Fourteen of the treated patients had disease that resolved or abated (mean survival +/- SE, 73.7 weeks +/- 14.6 weeks), and 13 had disease that persisted unchanged or worsened (mean survival +/- SE, 57.3 +/- 15.8 weeks). The outcome was poor for 17 patients who did not receive effective therapy (mean survival +/- SE, 14.1 +/- 5.3 weeks). M. kansasii infection presents late in the course of HIV disease, and the lung is the organ most frequently involved. Survival is clearly influenced by therapy, and even patients who respond poorly to therapy survive longer than those who are not treated.