This study aims to evaluate the effect of weight loss on the physical capacity and the structure and function of the heart after bariatric surgery. Forty-three adult obese patients, 31 women (72.1%) and 12 men (27.9%), were submitted to electrocardiogram, ergometric test, and echo Doppler cardiogram at presurgical stage and 1 year after Roux-en-Y gastric bypass. The statistical analysis utilized the Wilcoxon, Student's t, and Shapiro-Wilk tests with 5% significance level. Weight was reduced from 116.5 +/- 21.5 to 80 +/- 15.9 kg and body mass index from 41.8 +/- 4.4 to 28.4 +/- 3.8 kg/m(2). Cardiac frequency was reduced from 77.9 +/- 9.6 to 70.9 +/- 7.8 bpm, systolic pressure from 130 +/- 20 to 120 +/- 10 mmHg, and diastolic pressure from 80 +/- 10 to 80 +/- 0 mmHg. Ergometric tests showed distance covered from 378.9 +/- 126.5 to 595 +/- 140.4 m, metabolic coefficient from 6.7 +/- 2.4 to 8.3 +/- 2.6 ml O-2/kg/min, and oxygen consumption (VO2) from 23.1 +/- 8.4 to 30 +/- 10.3 metabolic equivalents of task. The echo Doppler cardiogram showed interventricular septum from 12 +/- 2 to 10 +/- 1 mm, posterior wall from 11 +/- 2 to 10 +/- 1 mm, and ventricular mass from 273 +/- 85 to 216 +/- 60 g. There was improvement in the diastolic function with an increase in the E'/A' and E/A relationship and the ejection fraction with an increase from 70.2 +/- 7.2% to 72.9 +/- 6.4%. Bariatric surgery improved physical capacity as well as structural and functional improvement of the heart.