OBJECTIVE The weight percentage of body fat in anorexic adolescents on the basis of four distinct formulas (Siri; Lohman; Westrate and Deurenberg; Heyward and Stolarczyk) is compared. METHOD The body composition of 238 anorexia nervosa (AN) patients, divided into four age categories (13-15, 15-17, 17-19, and 19-22 years), was measured by means of densitometry (underwater weighing). RESULTS Depending on the formula, the results calculated by the formulas differ significantly in each age category. The Siri formula generally results in the highest mean fat percentage (12.76%-13.39%) whereas the Heyward and Stolarczyck formula shows the lowest figures (8.77%-9.31%). Applying the Lohman and the Heyward and Stolarczyck formulas to the 19-22 years category, results in negative fat percentages. CONCLUSION A clinically useful formula is important for the estimation of body composition in specific age categories. We recommend with some restrictions, to apply the Siri formula in AN patients.