The object of the investigation was to compare the effect of three different physiotherapy masks on the incidence of postoperative complications after thoracic surgery. It was carried out as a prospective, consecutive, randomized comparison at a Department of Thoracic and Heart Surgery at a University Hospital. The therapy was performed by experienced and specially trained physiotherapists. One hundred and sixty patients were evaluated; 60 patients undergoing heart surgery, 59 patients having pulmonary resection, and 41 patients with exploratory thoracotomy. In each operative category the patients were treated with one of three face mask systems used in addition to routine chest physiotherapy. These were either continuous positive airway pressure (CPAP), positive expiratory pressure (PEP), or inspiratory resistance--positive expiratory pressure (IR-PEP). Postoperative pulmonary complications were assessed by forced vital capacity (FVC), arterial oxygen tension (PaO2), and chest X-ray examination, all parameters were measured preoperatively and on the fourth and ninth postoperative day. The patients filled in a questionnaire concerning their opinions about their mask treatment. IR-PEP showed a lesser decrease in PaO2 on day nine. Otherwise there was an equal decrease in FVC and PaO2, and equal frequency of atelectasis in the three mask treatments. It is therefore concluded that any of the three therapies: continuous positive airway pressure (CPAP), positive expiratory pressure (PEP), and inspiratory resistance--positive expiratory pressure (IR-PEP) may be used as supplement to standard chest physiotherapy.