Relationship between weight and hypothalamic-pituitary-ovarian axis has been long time studied. Underweight female patients can have or not disturbances of gonadal system and the importance of the weight or body composition in the mechanisms of these abnormalities is not clear. In our study, we have investigated body composition in 15 underweight female patients. Five patients have had anorexia nervosa, with amenorrhea (group AN), and other 10 patients were eumenorrheic, but with low body weight (hypoanabolic syndrome--group HA). Bioelectrical impedance method was used to investigate body composition. Total body water (TBW), lean mass (LM), and fat mass (FM) were determined. Nutritional status was also assessed using body mass index (BMI). The mean body weight was significantly decreased (p = 0.05, 36.6 +/- 4.68 Kg versus 44.1 +/- 4.67 Kg) in group AN as compared to group HA. Fat mass was very low in group AN (9.45% of body weight versus 11.3% of body weight in group HA), but there was not significant difference. Lean mass was significantly decreased in group AN (33.24 +/- 2.37 Kg) in comparison with group HA (39.3 +/- 4.06 Kg). Correlations between anthropometric indices were studied. Positive significant correlations were obtained between body weight and lean mass, total body water and basal metabolism in both groups. In conclusion, our study reveals that body weight has a relative importance in ovarian axis dysfunction, and the most important of body compartments seems to be lean mass.