Pulmonary function and pulmonary gas exchange at rest, and during and after a standard exercise load of 500 kpm in 1 min on bicycle ergometer were studied in 34 women with severe, uncomplicated obesity, aged 37.8 (20-59) years, before and 1 year after gastric banding, resulting in a weight loss from 113.2 (84-156) to 81.7 (60-110) kg. Following the weight loss, TLC and VC rose from 93 and 94 per cent of expected to 98 and 101 per cent, respectively. FRC, ERV and FRC/TLC rose more markedly from 77, 64 and 83 per cent to 98, 109 and 99 per cent. IC fell from 108 to 99 per cent. RV and RV/TLV remained unchanged. FEV1.0 rose from 97 to 103 per cent, while MVV rose from 102 to 112 per cent, i.e. above normal. TLCO and PaCO2 remained unchanged, at 90 and 95 per cent, whereas PaO2 rose from 86 to 91 per cent. Resting O2 intake (VO2) decreased from 147 to 115 per cent of the expected for normal weight women, while VO2/BSA decreased from 113 to 99 per cent, the changes being greater than expected from commonly used formulas for prediction of metabolic rate. O2 cost of work (EO2) decreased from 142 to 105 per cent. Resting ventilation (V) declined from 136 to 113 per cent, while ventilatory cost of work (EV) decreased from 142 to 105 per cent. CO2 recovery time after work (CO2RT) decreased from 121 to 100 per cent, while the ratios CO2RT to EO2 and to extra CO2 output of work (ECO2) rose slightly. Thus, the loss of weight led to increased filling of the lungs, improved dynamic function, reduced ventilation/perfusion disturbances and greater than expected reduction of energy expenditure, both at rest and exercise. In the obese state there was no evidence of alveolar hypoventilation or impaired ventilatory control. The beneficial effect of weight reduction on the exertional dyspnea included a combination of marked reduction of ventilatory demands and moderate rise in ventilatory capacity.