The contribution of disseminated Mycobacterium avium complex (DMAC) infection to the morbidity and mortality of patients with acquired immune deficiency syndrome (AIDS) is unclear. Previous studies that suggested the decreased survival of patients with AIDS and DMAC had incomplete information on patient immunologic status and follow-up. We studied patients with AIDS and DMAC and compared their survival with that of AIDS patients without DMAC but with other comparable risk factors for survival. Case and control subjects were similar in terms of CD4 cell count, prior AIDS status, history of antiretroviral therapy, history of Pneumocystis carinii prophylaxis, and year of diagnosis. A group of 39 patients with untreated DMAC had significantly shorter survival, mean of 5.6 +/- 1.1 months (median 4 months), than 39 matched patients with AIDS but without DMAC, mean 10.8 +/- 1.3 months (median 11 months, p less than 0.0001). The survival of 16 additional patients with DMAC who received antimycobacterial therapy, mean of 9.5 +/- 1.4 months (median 8 months), was not significantly shorter than that of an additional 16 matched control subjects, mean 11.7 +/- 1.9 months (median 11 months, p = 0.58). Patients with treated DMAC survived significantly longer than those with untreated DMAC (p less than 0.01). We conclude that untreated DMAC significantly shortens survival. Moreover, these results indicate that patients with DMAC who receive antimycobacterial therapy do not experience the shortened survival seen in untreated DMAC.