We report the transient spontaneous disappearance of a mucocele due to bronchial atresia. Two years before presentation, a chest radiograph showed a hyperlucent right upper lung and a mucocele near the right hilum. A chest radiograph taken 1 year later showed that the mucocele had disappeared leaving an ovoid outline of a dilated bronchus. A chest radiograph obtained 3 months before presentation showed that the mucocele was present again. Atresia of the B3b bronchus of the right upper lobe was noted on thoracotomy. The "disappearance" of the mucocele probably was due to the clearance of mucoid material through collateral airways.