<i>Background:</i> Weight loss has been shown effective in the treatment of the obstructive sleep apnea-hypopnea syndrome. Regrettably, many obese patients are unable to achieve sustained and useful weight loss by dietary means. Recently, bariatric surgery has emerged as an alternative to treat obesity and many of its comorbidities, although its role for sleep apnea treatment is still not defined. <i>Objectives:</i> To evaluate the impact of bariatric surgery on obstructive sleep apnea in morbidly obese patients. <i>Methods:</i> In this cohort study, polysomnography, Epworth Sleepiness Scale questionnaire and clinical assessment were performed in 12 of 13 morbidly obese patients with moderate to severe obstructive sleep apnea treated with bariatric surgery through Roux-en-Y gastric bypass procedure after a minimum of 18 months post surgery. <i>Results:</i> The mean (±SD) loss of excess body weight was 70.5 ± 24%. The mean level obtained in the Epworth Scale was 4.8. There was a significant reduction in the apnea-hypopnea index, from a median of 46.5 (range: 33–140) to 16 (range: 0.9–87) events per hour (p < 0.05), an improvement in mean oxygen saturation from 85.7 ± 5.1 to 94.5 ± 3.6% (p < 0.05) and in minimum oxygen saturation from 64.7 ± 13.4 to 78.7 ± 13.7% (p < 0.05). The magnitude of the weight loss and the improvements in mean and minimum oxygen saturation were positively correlated, (r = 0.76; p ≤ 0.05, and r = 0.59; p ≤ 0.05, respectively). <i>Conclusions:</i> Weight loss achieved by bariatric surgery is associated with significant long-term improvements in obstructive respiratory event, oxygenation and resolution of daytime somnolence.