It is currently recommended that patients with AIDS and Mycobacterium avium complex (MAC) bacteremia receive antimycobacterial treatment. However, no study has prospectively evaluated the impact of this infection and its treatment on survival. This study prospectively followed a cohort of 367 AIDS patients with < or = 50 CD4+ cells/microL and found that MAC bacteremia was independently associated with an increased risk of death (relative hazard [RH] = 1.8, 95% confidence interval [CI] = 1.3-2.4, P < .001). Patients with MAC bacteremia who were treated had a longer median survival than those who were not (263 vs. 139 days, P < .001); treatment was independently associated with a lower risk of death (RH = 0.45, 95% CI = 0.23-0.89, P < .001). However, 23% of patients with bacteremia died within 28 days of that diagnosis; few were treated. MAC bacteremia contributes to the death of patients with AIDS, and treatment increases survival. However, many patients will not survive long enough to receive treatment. These results underscore the importance of early diagnosis and chemoprophylaxis for MAC bacteremia.