BACKGROUND Morbidly obese subjects are known to have impaired respiratory function and inefficient respiratory muscles. A study was undertaken to investigate the influence of excessive weight loss on pulmonary and respiratory muscle function in morbidly obese individuals who underwent gastroplasty to induce weight loss. METHODS Twenty one obese individuals with mean (SE) body mass index (BMI) 41.5 (4.5) kg/m2 without overt obstructive airways disease (FEV1/FVC ratio > 80%) were studied before and six months after vertical banded gastroplasty. Only patients who had lost at least 20% of baseline BMI were included in the study. Standard pulmonary function tests and respiratory muscle strength and endurance were measured. RESULTS Before operation the predominant abnormalities in respiratory function were significant reductions in lung volumes and respiratory muscle endurance and, to a lesser degree, reductions in respiratory muscle strength. All parameters increased towards normal values after weight loss with significant increases in functional residual capacity (FRC) from 84.0 (2.2) to 91.3 (2.5)% of predicted normal values (mean difference 7.3, 95% confidence interval of difference (CI) 4.2 to 10.5), total lung capacity (TLC) from 85.6 (3.0) to 93.5 (3.7)% of predicted normal values (mean difference 7.9, 95% CI 4.5 to 11.5), residual volume (RV) from 86.7 (3.1) to 96.4 (3.0) of predicted normal values (mean difference 9.7, 95% CI 5.2 to 14.1), expiratory reserve volume (ERV) from 76.6 (3.0) to 89.0 (3.4)% of predicted normal values (mean difference 12.4, 95%, CI 6.3 to 18.9), respiratory muscle strength: PImax from 92 (4.4) to 113 (4.6) cm H2O (mean difference 21, 95% CI 12.2 to 31.6), PEmax from 144 (5.6) to 166 (4.3) cm H2O (mean difference 22, 95% CI 12.9 to 32.0), and endurance: PmPeak/PImax from 56 (1.4) to 69 (2.0)% (mean difference 13, 95% CI 9.7 to 16.9). The strongest correlation was between weight loss and the improvement in respiratory muscle endurance. CONCLUSIONS Lung volumes and respiratory muscle performance are decreased in obese individuals. Weight loss following gastroplasty is associated with improvement in lung volumes and respiratory muscle function.