Atelectasis has been reported as a common cause of fever in the first 48 hours after surgery. A group of one hundred patients programmed for elective abdominal surgery were studied with chest roentgenograms, both before and 48 hours after surgery. Thirty-one of them developed atelectasis and eighteen developed fever. Four of the patients with and fourteen without atelectasis, had fever. Five cases had unexplained fever, three of them with pulmonary atelectasis. These findings suggest that atelectasis can cause postoperative fever, but it is not the most common cause of fever in the first 48 hours after surgery.