Analysis of geographic risk was performed for Mycobacterium avium complex (MAC) bacteremia among North American patients with AIDS. Monthly mycobacterial blood cultures were taken from patients who were placebo recipients in a prospective evaluation of MAC prophylaxis. Of 571 patients, 102 (17.9%) acquired MAC bacteremia during an average follow-up of 256 days. The area with the highest risk for MAC was the South Central region (27.9%; P