We analyzed cases of disseminated nontuberculous mycobacterial infection (DNTM) in patients with AIDS reported to the Centers for Disease Control. Between 1981 and 1987, 2,269 cases were reported. In 96% of cases, infection was caused by M. avium complex (MAC). The number of cases has risen steadily since 1981, but the rate as a percentage of AIDS cases has remained stable at 5.5%. DNTM was seen less frequently in AIDS cases with Kaposi's sarcoma than in other AIDS cases (p less than 0.01). Rates of DNTM were lower in Hispanics and declined with age but were not significantly different by patient sex or means of acquiring HIV infection. Rates of disseminated MAC varied by geographic region from 3.9% to 7.8% (p less than 0.0001). As assessed by helper/suppressor T-cell ratios, AIDS patients with DNTM were not more immunologically impaired than those with other opportunistic infections. Life table analysis revealed that AIDS patients with DNTM survived a shorter time (median, 7.4 months) than did other AIDS patients (median, 13.3 months; p less than 0.0001). We conclude that DNTM is acquired by unpreventable environmental exposures. Because DNTM adversely affects survival of AIDS patients, effective therapeutic agents must be vigorously sought.