BACKGROUND obesity impacts on respiratory function and also it acts as a risk factor for obstructive sleep apnea (OSA). OBJECTIVE to study the effects of bariatric surgery on pulmonary function tests and on OSA in morbidly obese women over 4 years. METHODS fifteen morbidly obese women (mean body mass index [BMI] 50.52 ± 12.71 kg.m-2, mean age 40.13 ± 10.06 years) underwent pulmonary function tests (PFT) in two opportunities (before and after weight loss surgery). PFT included spirometry, body plethysmography and measure of maximal inspiratory mouth pressure (PImax) and of tension-time index for inspiratory muscles. Also, in both opportunities, resting arterial blood gas tensions were evaluated and a full night sleep register was performed. RESULTS BMI significantly decreased after bariatric surgery (-44.07 kg.m-2 [CI 95% -38.32 - -49.81]). Also, there was a significantly increase in forced expiratory volume in 1 second (FEV1) (p < 0.01), forced vital capacity (FVC) (p < 0.01), expiratory reserve volume (ERV) (p = 0.040), functional residual capacity (FRC) (p = 0.009) and a decline in airways resistance (Raw) (p = 0.018). Concerning sleep registers, apnea hypopnea index (p = 0.001) and desaturation index (p = 0.001) were also reduced after weight loss. Improve in ERV had a significant correlation with weight loss (r = 0.774, p = 0.024). Conclussions: pulmonary function tests and apnea hypopnea index improve after bariatric surgery in mor bidly obese women. Improvement of ERV is well correlated with weight loss.