We investigated interstitial pneumonia, one of the most severe postoperative acute complications of respiratory surgery. In sixteen years, from 1982 through 1997, 2,453 cases underwent thoracotomies caused by lung cancers, in our institute. There were 20 cases (0.79%) who died within 30 days after surgery (operative death). Six out of 20 cases died of interstitial pneumonia, one of the most frequent postoperative complications. We examined these six patients and 2 patients with postoperative interstitial pneumonia who died within 32 days. Preoperative lung function tests, laboratory data, chest CT and/or chest X-ray films cannot predict development of postoperative interstitial pneumonia. Those who acquired postoperative interstitial pneumonia had continuous high fever after surgery following reticular shadows on their chest X-ray and increase in LDH levels. We could not detect any characteristic viral infections. Although, they were treated by steroid, elastase inhibitor and/or immune suppressant, they died within 32 days. In conclusion, development of postoperative of interstitial pneumonia cannot be predicted from preoperative laboratory data and/or lung function tests.