A 51-year-old man was found to have cancer in the left lung during treatment for idiopathic interstitial pneumonia. He underwent a left upper lobectomy because pulmonary function was judged to be adequate and because the interstitial pneumonia was not severe. After the operation, acute exacerbation of interstitial pneumonia occurred, and respiratory failure developed. Immediate steroid therapy alleviated his condition. Acute exacerbations of idiopathic interstitial pneumonia after pulmonary resection are usually fatal, and cases such as this are very rare.