The goals of the study were to determine if moderate weight loss in severely obese adults resulted in (i) reduction in apnea/hypopnea index (AHI), (ii) improved pharyngeal patency, (iii) reduced total body oxygen consumption (VO(2)) and carbon dioxide production (VCO(2)) during sleep, and (iv) improved sleep quality. The main outcome was the change in AHI from before to after weight loss. Fourteen severely obese (BMI > 40 kg/m(2)) patients (3 males, 11 females) completed a highly controlled weight reduction program which included 3 months of weight loss and 3 months of weight maintenance. At baseline and postweight loss, patients underwent pulmonary function testing, polysomnography, and magnetic resonance imaging (MRI) to assess neck morphology. Weight decreased from 134 +/-6.6 kg to 118 +/- 6.1 kg (mean +/- s.e.m.; F = 113.763, P < 0.0001). There was a significant reduction in the AHI between baseline and postweight loss (subject, F = 11.11, P = 0.007). Moreover, patients with worse sleep-disordered breathing (SDB) at baseline had the greatest improvements in AHI (group, F = 9.00, P = 0.005). Reductions in VO(2) (285 +/- 12 to 234 +/-16 ml/min; F = 24.85, P < 0.0001) and VCO(2) (231 +/- 9 to 186 +/- 12 ml/min; F = 27.74, P < 0.0001) were also observed, and pulmonary function testing showed improvements in spirometry parameters. Sleep studies revealed improved minimum oxygen saturation (minSaO(2)) (83.4 +/- 61.9% to 89.1 +/- 1.2%; F = 7.59, P = 0.016), and mean SaO(2) (90.4 +/- 1.1% to 93.8 +/- 1.0%; F = 6.89, P = 0.022), and a significant increase in the number of arousals (8.1 +/- 1.4 at baseline, to 17.1 +/- 3.0 after weight loss; F = 18.13, P = 0.001). In severely obese patients, even moderate weight loss (approximately 10%) boasts substantial benefit in terms of the severity of SDB and sleep dynamics.