The patient was a 74-year-old man who had undergone surgery for rectal cancer 9 years before and had developed left lung metastasis(S3)3 years and 4 months prior to admission. He had received video assisted left lung wedge resection. He presented with a growing nodular lesion close to the remaining left lung margin and elevated serum carcinoembryonic antigen(CEA)levels, and underwent open extended segmentectomy. The chest drain tube was removed on 3rd post-operative day, but he developed left pneumothorax on 4th post-operative day and a computed tomography(CT)scan revealed a cystic lesion 5.0 cm in size at the base of his left lung. Revision surgery was performed on 8th post-operative day. A pulmonary cyst on the diaphragmatic surface of the lung(S10)was found and location of the air leak was confirmed in the same area. Following wedge resection of the cyst-containing region, the leak ceased completely. Rapid manifestation of a newly formed pulmonary cyst during the acute post-operative period is rare.