We investigated the effects of providing patients with continuous positive airway pressure during a short weaning period from mechanical ventilation to extubation. Following elective primary aortocoronary bypass surgery employing cardiopulmonary bypass, 80 patients received intermittent positive pressure ventilation for 5 h. At the end of this time patients were randomly allocated to one of the following three groups for a 2 h period of spontaneous respiration prior to extubation: group CPAP0 = T-piece circuit (n = 27); group CPAP5 = 5 cmH2O of continuous positive airway pressure (n = 27); group CPAP10 = 10 cmH2O of continuous positive airway pressure (n = 26). Following extubation, pulmonary gas exchange was assessed after 1, 2, 4, and 24 h. All patients had good pre-operative lung function. There was a significant increase in the median alveolar-arterial oxygen partial pressure difference and a decrease in the oxygenation index in all three groups during the period of added inspired oxygen which persisted until 24 h post-extubation, but there were no differences between the groups. In patients with good pre-operative lung function requiring primary aortocoronary bypass surgery, the use of continuous positive airway pressure confers no advantage over a simple T-piece attachment during the short period of weaning from mechanical ventilation to extubation.