We reported 2 cases of Mycobacterium avium complex lung disease occurred in middle-aged women without underlying disorders, which we could observe for more than 30 years. One case was a 42-year-old woman started with bloody sputum, and the other was a 43-year-old woman with cough and sputum. In both cases, chest X ray films were normal on their first visit. More than 15 years after their first visit, Mycobacterium avium complex was isolated from their sputum or bronchial washing. During the observation, a cluster of small nodules in the periphery of the lung and bronchiectasis appeared and deteriorated, and excretion of the bacilli increased gradually. Their past history and family history were normal. Since lung disease caused by Mycobacterium avium complex progresses very slowly, long-time observation would be necessary to consider its pathogenesis.