We conducted a retrospective study to further elucidate the clinical presentations and prognosis of disease due to Mycobacterium kansasii in patients infected with human immunodeficiency virus (HIV). Forty-nine HIV-infected patients first had M. kansasii isolated at a mean CD4 cell count of 62/mm3 and at a mean interval of 17 months after the diagnosis of AIDS. Seventeen of the 49 patients had disseminated disease caused by M. kansasii. Twenty-nine patients had a positive acid-fast smear of sputum, and 35 were known to be cigarette smokers. At the time of initial isolation of M. kansasii, 13 patients had other concurrent pulmonary isolates and 15 had another mycobacterial species concurrently isolated (the Mycobacterium avium complex in 13 instances). Patients who received antimycobacterial treatment survived longer than those who did not. Only one of the 49 patients was definitively determined to be colonized with M. kansasii without disease; therefore, it appears that pulmonary isolates of M. kansasii in HIV-infected patients are almost always associated with disease. The increase in rates of M. kansasii disease among HIV-infected patients has paralleled the rise of AIDS in Louisiana. So far, this state has recorded more coinfections with M. kansasii and HIV than any other.