Malnutrition is associated with a number of systemic diseases that are often accompanied by severe exercise limitation. Anorexia nervosa (AN) is a disease characterized by malnutrition due to psychological factors rather than systemic disease. Diminished exercise capacity in AN has been attributed to a loss of muscle mass, dysfunction of remaining muscle, and impaired cardiovascular responses. In order to evaluate the role of malnutrition in the cardiopulmonary response to exercise, nine adolescent girls with AN were evaluated during progressive and steady-state exercise testing using a cycle ergometer. Nutritional status was assessed by body mass percentile (BMP) and percent ideal weight (PIWT). Cardiac output was measured by the indirect (CO2 rebreathing) Fick method. Maximum work capacity (Wmax) was expressed as a percent of predicted for sex and height, and cardiac output as a percent of predicted for oxygen consumption. To ensure that the laboratory values were comparable to the predicted values, a control group consisting of ten adolescents was studied concurrently. Wmax was below the 95% confidence interval in six of nine of the AN group (mean ± SD; 70 ± 22% predicted), whereas two of ten controls were below and one above this interval (112 ± 37%). Wmax correlated with nutritional status (BMP: r = 0.75; P < 0.001; PIWT: r = 0.8, P < 0.001). Ventilatory responses for CO2 production at steady state and for Wmax were appropriate in both groups. Cardiac output was appropriate in both the controls (103 ± 12%) and the AN group (104 ± 14%). This was accomplished in the AN group with a relatively low stroke volume (86 ± 19%) and high heart rate (107 ± 13%), both expressed as percent predicted for sex and work. With appropriate cardiac and ventilatory responses, exercise capacity in AN appears to be limited primarily by diminished muscle mass and/or dysfunction of the existing muscle. © 1992 Wiley-Liss, Inc.