Obesity is associated with an increased risk of sudden death that may be due to abnormal cardiac vagal modulation reflected by reduced heart rate variability (HRV). Few studies have been conducted analyzing the effect of bariatric surgery-induced weight loss on HRV assessed by 24-hour Holter monitoring. The aim of this study was to assess weight loss effect after bariatric surgery on HRV and ventricular size and function. Ten morbidly obese patients, 6 women and 4 men aged 24 to 47 years, underwent bariatric surgery. Seven morbidly obese patients without active obesity treatment were used as controls. Twenty-four-hour Holter monitoring and echocardiogram were obtained before and at 6 to 12 months after surgery or at follow-up in control patients. Changes in minimal, maximal, and mean heart rate along with HRV during daytime and nighttime were compared before and after surgery. Baseline characteristics in the control group did not differ significantly from the treatment group. Average weight in the treatment group was 141 +/- 31 kg (mean +/- SD) at baseline and decreased to 101 +/- 18 kg at follow-up, corresponding to a body mass index of 52.3 +/- 7.6 kg/m(2) at baseline and 37.7 +/- 5.3 kg/m(2) at follow-up. There was a decrease in minimal heart rate (48 +/- 10 vs 40 +/- 6 beats per minute, P = .021) and mean heart rate (82 +/- 7 vs 66 +/- 10 beats per minute, P < .001) during the Holter monitoring. Spectral analysis showed a significant enhancement in HRV parameters (high- and low-frequency power) because there was an increase in the standard deviation of normal to normal R-R intervals (116 +/- 25 vs 174 +/- 56 milliseconds, P < .001), the standard deviation of the mean R-R intervals calculated over a 5-minute period (104 +/- 25 vs 148 +/- 45 milliseconds, P < .001), the square root of the mean of the squared differences between adjacent normal R-R intervals (25 +/- 8 vs 50 +/- 20 milliseconds, P < .001), and the percentage of differences between adjacent normal R-R intervals exceeding 50 milliseconds (5% +/- 5% vs 22% +/- 13%, P < .001). Echocardiographic measures remained unchanged when comparing the groups. Weight loss after bariatric surgery enhances HRV and decreases mean and minimal heart rate during Holter monitoring through a better cardiac parasympathetic modulation.