One hundred consecutive cases of pulmonary infection due to Mycobacterium intracellulare-avium seen during a 3 1/2-year period qualified for review on the basis of a compatible chest x-ray film, repeated isolations from cultures of sputum, and follow-up of three to eight years. Infections with M intracellulare-avium represented 27 percent of all mycobacterial infections seen during this period, including those due to M tuberculosis. The cases of disease due to M intracellulare-avium were predominantly in men with preexisting pulmonary disease, with a peak incidence in the sixth decade, but nearly one-third of the cases were in younger persons free of coexisting disease. The disease was chronic and indolent in most cases, and only a few showed a progressive course. A stable course was frequently observed despite prolonged persistently positive cultures of sputum. A favorable prognosis was most often found in those with previously treated tuberculosis. Poor prognosis was often due to a serious associated disease, such as cancer, rather than to advancing mycobacterial infection itself. Age, sex, or race was unrelated to prognosis. Conversion to negative status on culture was attained in one-half of the cases. Those with extensive radiographic involvement or cavitation were more likely to have treatment fail bacteriologically. No combination of chemotherapy appeared to be particularly effective, including the use of five or more drugs in eight cases demonstrating progressive disease. Surgery, too, was ultimately disappointing in that recurrence appeared in six of 18 carefully selected cases.