Four patients with Mycobacterium kansaii pulmonary infection were followed without treatment for 10 to 14 years after diagnosis. Although spontaneous resolution of active disease occurred 5 years after diagnosis in one patient, slowly progressive disease in the absence of significant symptoms was documented in 3 patients during a 12-to-14-year follow-up period. Administration of antituberculous drugs resulted in rapid resolution of signs of active disease in these patients. These observations added to our limited knowledge of the natural history of M. kansasii disease.