The aim of this study was to evaluate exercise performance in patients affected by anorexia nervosa. We studied 19 patients (all females, mean age 23.1 +/- 5.2 years) affected by anorexia nervosa (mean weight 37.3 kg, body mass index 14.04 +/- 1.4 kg/m2) and 20 constitutionally thin women, matched for age, height and physical activity, with a body mass index < 19 kg/m2. All these women underwent clinical examination, standard ECG and a cardiopulmonary stress test. Patients affected by anorexia nervosa showed a lower heart rate and systolic blood pressure at peak exercise (148.8 +/- 13.8 vs 171 +/- 9.2 b/min, p < 0.001, and 130 +/- 9.5 vs 152 +/- 11.2 mmHg, p < 0.001), work load (85.5 +/- 15.1 vs 117.2 +/- 20.3 W, p < 0.001), rate-pressure product (19 371 +/- 2391 vs 25,986 +/- 2218 b/min/mmHg, p < 0.001), oxygen uptake (VO2) at rest and maximum VO2 (5.4 +/- 1.7 vs 7.1 +/- 1.1 ml/kg/min, p < 0.001, and 28.08 +/- 6.3 vs 40.2 +/- 7.1 ml/kg/min, p < 0.001), anaerobic threshold (15.7 +/- 1.9 vs 20.4 +/- 2.1 ml/kg/min, p < 0.001), VO2 during exercise (9.5 +/- 1.2 vs 12.8 +/- 1.3 ml/min/W, p < 0.001), maximum minute ventilation (34.5 +/- 9.9 vs 48.4 +/- 10.3 /min, p < 0.001), and oxygen pulse (7.2 +/- 2 vs 10.9 +/- 2.4 ml/b, p < 0.001). These data show an abnormal working capacity and cardiovascular responses to exercise in patients affected by anorexia nervosa. The low VO2, both at rest and during exercise, allows them to maintain a relatively high level of physical activity, which contributes to increase the energy expenditure needed for weight loss.