1. The prevalence of cardiac autonomic alterations was evaluated in 23 obese subjects with body mass index 37.2 ± 3.03 kg/m 2 (mean ± sd ), compared with 78 controls with body mass index 22.5 ± 2.6 kg/m 2 ( P < 0.001). 2. Cardiac autonomic function was assessed by four standard tests (heart rate response to deep breathing and to the Valsalva manoeuvre, systolic blood pressure fall after standing and diastolic pressure rise during handgrip) and by the cross-correlation test, a new method of computerized analysis of respiratory sinus arrhythmia based on spectral analysis of electrocardiographic and respiratory signals. 3. Considering tests indicative of parasympathetic function, only the heart rate response to the deep breathing and the cross-correlation test were significantly lower in the obese than in the control group [deep breathing = 13.95 ± 8.65 beats/min (mean ± sd ) vs 24.5 ± 7.65, P < 0.001; cross-correlation 4.28 ± 0.74 units vs 5.14 ± 0.63, P < 0.001]. Deep breathing and/or cross-correlation were abnormal in 10 (43.5%) obese subjects (deep breathing: seven subjects, cross-correlation: eight subjects). No significant difference between groups was found for the response to the Valsalva manoeuvre: the Valsalva ratio was 1.69 ± 0.45 in obese subjects and 1.88 ± 0.33 in controls ( P = NS). The Valsalva ratio was abnormal in three obese subjects. 4. No significant differences were found between groups for tests indicative of sympathetic function. The rise in diastolic blood pressure after handgrip was 12.6 ± 6.2 mmHg (1.67 ± 0.82 kPa) in obese subjects and 18.2 ± 4.9 mmHg (2.42 ± 0.65 kPa) in controls ( P = NS), and the fall in systolic blood pressure after standing was −6.8 ± 8.6 mmHg (−0.90 ± 1.14 kPa) in obese subjects and −6.9 ± 10.4 mmHg (−0.91 ± 1.38 kPa) in controls ( P = NS). The handgrip test was abnormal in four obese subjects, while no obese subject had an abnormal blood pressure response to standing. 5. Our findings suggest a high incidence of cardiac autonomic dysfunction in obese subjects. Since cardiac autonomic alterations have been shown to be involved in the mechanisms of cardiac sudden death, our data suggest a possible role of autonomic dysfunction in the increased risk for sudden death in obesity.