Body mass index (BMI) fails to detect altered nutritional state in the presence of overweight or obesity, since malnutrition can be present and masked by the abnormal amount of fat mass. Measuring body cell mass (BCM) contents for the evaluation of muscle mass and protein tissue states is well accepted. The aim of the present study was to apply body cell mass index (BCMI) to monitor the muscular mass changes of male and female Olympic athletes, renal dialysis patients, and anorexia nervosa patients in comparison with healthy subjects. The BCMI values of male subjects from the healthy group and Olympic athletes groups, but not the renal dialysis group, were significantly higher ( p<0.0001) than those of female subjects from the same groups. In addition, subjects with normal or high BMI values may be malnourished as highlighted by a low BCMI. We believe the BCMI is more sensitive than the BMI for studying the nutritional status of the individual.