Treatment with intermittent positive pressure breathing (IPPB) and incentive spirometry (I.S.) was compared in 109 patients after heart surgery with cardiopulmonary bypass. Assessment was done by measurement of vital capacity, arterial oxygen tension and identification of the radiological signs of atelectasis. All patients were instructed pre-operatively in the treatment which was to be used. Vital capacity, arterial oxygen tension while breathing air for the first three postoperative days and the incidence of atelectasis showed no significant difference between groups. Ten minutes after treatment the arterial oxygen tension fell, but this was only statistically significant after I.P.P.B. At 60 minutes the arterial oxygen tension had returned to pretreatment level in both groups. The use of the incentive spirometer four times daily is no better than I.P.P.B., in preventing atelectasis after open-heart surgery. Possibly incentive spirometer treatment given more frequently may be more effective.