Pneumothorax in patients with pulmonary Mycobacterium avium complex (MAC) disease is considered to be a rare complication, and little is known about its clinical course. In this study, we aimed to define the clinical features, outcome, and prevalence of pneumothorax in patients with pulmonary MAC disease. A retrospective review of medical records identified eight men and ten women (mean age, 75 years) with active pulmonary MAC disease complicated by pneumothorax between 2003 and 2010 in our institution. None of the patients was positive for HIV infection. Pneumothorax occurred in the right lung in 12 patients and in the left in six. All but one patient had MAC disease in both lungs, and 12 patients had widespread lesions covering a total area larger than one lung field. Seven of the 18 patients (39 %) were forced to undergo surgery following unsuccessful thoracic drainage. Five patients experienced recurrence during the study period and two others eventually developed chronic pneumothorax. The complication rate of pneumothorax was calculated on the bases of the total number of patients with active pulmonary MAC disease during the same period. The overall complication rate of pneumothorax was as high as 2.4 % (18 of 746 patients with MAC disease). In conclusion, the incidence of pneumothorax in patients with active pulmonary MAC disease was unexpectedly high, especially in patients who were elderly and had advanced MAC disease. This condition is often difficult to treat and can recur easily.