During eleven and one-half years, 122 patients with M. intracellulare disease were treated at National Jewish Hospital and Research Center, Denver, Colorado. Of the 81 patients treated with multiple drug chemotherapy, 63 (78%) were considered to be treatment successes. On follow-up (averaging 55 months) 40 of the 63 patients had remained clinically stable or improved and 12 had grown worse. Ten of these 63 patients had relapsed. Of the 63 patients considered to be treatment successes, 23 (36.5%) had died during follow-up, mostly of pulmonary disease. Of the 18 patients who failed to respond to chemotherapy, 2 were later successfully treated with surgery, 5 patients were clinically worsening, and 8 (44%) had died during a follow-up period averaging 43 months. A group of 41 patients had indeterminate results with chemotherapy. Eight had died while in the hospital. Follow-up information after an average of 43 months indicated that 9 patients became treatment successes (5 with chemotherapy, 4 with surgery), 12 were clinically stable or improving, and an additional 13 had died for a total of 21 deaths (51%). The overall success of treatment in the 122 patients was 61%. Fifty-two (43%) died during the study. Many patients will respond to drug therapy and remain clinically well for years. Because of the potential seriousness of this disease in some patients, multiple drug chemotherapy should be used.