In 22 markedly obese, otherwise healthy, women studied postoperatively on each of days 1 through 3 after intra-abdominal operation, the effect of position change on arterial oxygenation was assessed. There was a statistically significant decrease in PaO2, an increase in A-aDO2 and a simultaneous modest reduction in PaCO2 with the assumption of the supine position on postoperative days 1 and 2, but not positional difference demonstrable in any studied variable by day 3. This report suggests that during the first 48 hours after abdominal surgery in obese patients, assumption and maintenance of the semirecumbent posture is a valuable therapeutic modality to improve arterial oxygenation.