We examined the long-term clinical course of mild pulmonary Mycobacterium avium complex (MAC) disease based on pulmonary changes detected on chest radiographs. The study involved 21 patients who could be followed up for more than 5 years, and their lesion extensions corresponded to extension type 1, according to the classification of pulmonary tuberculosis designated by the Japanese Society for Tuberculosis. These patients had no cavity or bronchial dilation. The percentage of patients with exacerbation was compared with those seen at the time of diagnosis and was found to be 5% after 1 year and 6% after 3 years. However, the percentages increased to 30% after 5 years and 67% after 8 to 10 years. Among the treated patients, the percentage of patients whose chest radiographic findings mildly improved or improved 1 to 2 years after treatment, as compared to their condition before treatment, which was 55%. Following that, however, the percentage of patients with mild improvements and normal improvement decreased over time, and that of patients with exacerbation increased. It is useful to know these clinical courses in examining patients with mild pulmonary MAC disease. Moreover, treatment efficacy needs to be sustained if the disease is treated.