OBJECTIVE To investigate the effect of mild-to-moderate obesity on respiratory mechanics at rest and during exercise in obese men. We hypothesized that the simple mass loading of obesity would alter both end-expiratory lung volume (EELV) and respiratory pressures (gastric, P(ga) and transpulmonary, P(TP)) in resting body positions and during graded cycle ergometry to exhaustion. METHODS A total of 10 obese (38+/-5% body fat; mean+/-s.d.) and nine lean (18+/-4%) men were studied. METHODS Body composition (by body circumferences and hydrostatic weighing) and pulmonary function were measured at rest. Breathing mechanics were measured at rest in the upright-seated position, supine, and during cycling exercise. Data were analyzed by independent t-test. RESULTS EELV was significantly lower in the obese men in the supine (30+/-4 vs 37+/-6% total lung capacity (TLC)) and seated (39+/-6 vs 47+/-5%TLC) positions and at ventilatory threshold (35+/-5 vs 45+/-7%TLC) (P<0.01). In contrast, at peak exercise, EELV was not different between groups. Respiratory pressures (P(ga) and P(TP)) were elevated (P<0.05) during one or more phases of the breathing cycle at rest and during exercise in obese men. CONCLUSIONS These data demonstrate that mild-to-moderate obesity in young men results in reduced lung volumes and alterations in respiratory mechanics when supine, seated at rest, and during exercise. During moderate exercise, obesity does not appear to limit changes in EELV; however, the regulation of EELV during heavy exercise appears to be affected.