Respiratory dysfunction is a well-recognized complication of cardiac operations. To quantify its current incidence and severity after uncomplicated cardiopulmonary bypass, serial measurements of arterial oxygen tension (Pao 2), alveolar-arterial oxygen gradient (Aao 2), and percentage pulmonary shunt fraction (%PSF) measured by a noninvasive technique were made in 129 patients (age, 59 ± 8 years (mean ± standard deviation) with good left ventricular function (left ventricular end-diastolic pressure <15 mm Hg) undergoing isolated coronary artery operations (group 1) and 30 patients undergoing general surgical procedures (group 2). Measurements were made before operation and on the first, second, and sixth postoperative days. Seven patients in group 1 who required prolonged ventilation were excluded from further study. In group 1, between the preoperative and second postoperative days, there was a marked fall in Pao 2 [89 ± 11 versus 57 ± 9 mm Hg; p < 0.001] and a marked increase in the Aao 2 gradient [18 ± 10 versus 50 ± 11 mm Hg; p < 0.001)] and %PSF [3 ± 1% versus 19 ± 6%; p < 0.001)] with only modest improvement by the sixth postoperative day [Pao 2, 67 ± 11 mm Hg; Aao 2, 45 ± 11 mm Hg; %PSF, 15 ± 41. There were similar but lesevere changes in Pao 2 and Aao 2 gradients in group 2 patients, with a return to baseline values by day 6. Regression analysis in group 1 patients showed a weak correlation between postoperative respiratory dysfunction and preoperative impairment of the Aao, gradient, but no correlation with age, sex, smoking status, New York Heart Association status, bypass time, or violation of the pleural sac(s). To determine the duration of respiratory dysfunction after cardiac surgery, serial Pao 2 and Aao 2 gradient measurements were continued until the sixth postoperative week in a further 30 patients (group 3). Group 3 patients demonstrated similar early impairment of respiratory function to group 1 patients but with complete resolution by the sixth postoperative week. This study demonstrates that respiratory dysfunction is both common and frequently severe even after uncomplicated cardiopulmcnary bypass but resolves by the sixth postoperative week. Respiratory dysfunction is also common after a major general operation but is less severe and resolves by the sixth postoperative day.