Body compartments can be assessed by means of bioelectrical impedance, a non-invasive, cost saving and simple physical method. The purpose of this study was to estimate the body compartments in anorexic patients, in the under-nutrition phase (group BMI < 17 kg/m2) and after weight restoration (BMI > 17 kg/m2). Total body water, intracellular and extracellular water have significantly decreased after weight restoration. The fat mass significantly (p < 0.001) increased in group BMI > 17 kg/m2 compared with group BMI < 17 kg/m2, and we have found a positive correlation between body mass index (BMI) and fat mass (r = 0.73, p < 0.0001). The assessment of free fat mass can provide information about the under-nutrition degree and the quality of re-feeding. The increase of free fat mass was significant (p < 0.001) in the group BMI > 17 kg/m2 as compare with the group BMI < 17 kg/m2). We have found a positive correlation between free fat mass and BMI (r = 0.73, p < 0.0001).