The patients with anorexia nervosa (AN) are known to be associated with high mortality, but the actual causes of death are still undefined. We tested the hypothesis that AN patients had cardiac disorders, including left ventricular (LV) dysfunction and LV atrophy. This study is a cross-sectional study at St. Luke’s International Hospital, Tokyo. We analyzed 13 female inpatients with AN. We assessed cardiac function and heart volume in AN by echocardiography, LV ejection fraction (LVEF), LV mass, and LV mass index (LVMI). We assessed the correlations between body mass index (BMI) and heart volume (LV mass and LVMI). The mean age and BMI were 34.8 ± 11.2 years and 15.5 ± 3.1 kg/m², respectively. There was no patient with mitral valve prolapse, but 3 patients had trivial to small amount of pericardial effusion. The mean LVEF was 67.7 ± 6.5%, and 12 out of 13 patients had normal LVEF. Their LV mass (89.0 ± 27.3 g) and LVMI (66.3 ± 16.4 g/m²) were small. BMI positively correlated with LVMI (r = 0.58, p = 0.040), as well as LV mass (r = 0.74, p = 0.004). Lower BMI reflects lower LVMI, as well as smaller LV mass. These issues suggest that heart volume is initially decreased in severe AN conditions. Low LVMI could be a good marker of severity of AN.