['Morbidly obese individuals (ie, body mass index [BMI], > or = 40 kg/m2) may have a pulmonary gas exchange impairment due to the large fat mass surrounding their abdomen.', 'To examine the effect of the waist-to-hip (W/H) ratio on pulmonary gas exchange in the morbidly obese.', 'Twenty-five morbidly obese individuals (mean [+/- SD] age, 39 +/- 10 years; mean BMI, 49 +/- 7 kg/m2; mean body fat, 50 +/- 6%; mean waist circumference, 135 +/- 15 cm; mean W/H ratio, 0.97 +/- 0.11) scheduled for bariatric surgery were recruited. Arterial blood was sampled in duplicate after 5 min of rest sitting upright.', 'The mean PaO2 at rest was 88 +/- 7 mm Hg (range, 72 to 108 mm Hg), the alveolar-arterial oxygen pressure difference (P[A-a]O2) was 19 +/- 9 mm Hg (range, 1 to 37 mm Hg), and the PacO2 was 38 +/- 3 mm Hg (range, 32 to 44 mm Hg). Linear regression showed that 32% and 36%, respectively, of the variance in the P(A-a)O2 and PaO2 were explained by the W/H ratio (p < 0.004 for both). As well, 20% of the variance in PacO2 was explained by the W/H ratio (p = 0.02). Men had larger W/H ratios (p < 0.01) and poorer gas exchange (p = 0.06) compared to women (mean difference: PaO2, -7 mm Hg; P[A-a]O2, 6 mm Hg).'] Is waist-to-hip ratio associated with pulmonary gas exchange in the morbidly obese?