To examine the prevalence of mitral valve prolapse (MVP) and to define the mechanisms of its development in patients with anorexia nervosa, we studied the cardiac function of 23 patients with anorexia nervosa by two-dimensional (2-D) echocardiography. MVP was present in 19 of 23 patients (82.6%), 10 having double MVP in both the anterior and posterior mitral leaflets and 9 having a single MVP in the anterior leaflet. There was no difference in echocardiographic parameters between the MVP(+) and MVP(-) groups. The heart rate was lower in the double MVP group (46.3 +/- 8.1; mean +/- SD) than in the non MVP group (64.0 +/- 6.3; p less than 0.01) and the single MVP group (55.9 +/- 10.9; p less than 0.05). The presence of bradycardia in these patients suggests an increase in vagal tone. The present study suggests that the incidence of MVP in anorexia nervosa is among the highest compared with its incidence in association with any other disorders, and an increase in vagal tone may be responsible for the development of MVP in patients with anorexia nervosa.