In 130 patients with no cardiac or pulmonary disease at the time of elective upper abdominal surgery, chest radiography was performed 2 and 4 days postoperatively. The sputum was examined and the patients were monitored with measurement of body temperature, arterial oxygen tension and white blood counts. Pneumonic infiltrates appeared in 8.4% of the patients and atelectasis in 68.5%. Most of the patients had elevated body temperature, raised white blood count and reduced arterial oxygen tension postoperatively. None of these commonly employed clinical or laboratory findings, singly or in combination, proved adequate for diagnosis of postoperative pneumonia.