Body fat and total body nitrogen (TBN) were quantified before and after refeeding in 32 female anorexia nervosa patients and in 29 matched control subjects by using the techniques of anthropometry and in vivo neutron-capture analysis (IVNCA). Mean body weight of patients (mean body mass index; BMI, in kg/m2), 15.4 +/- 1.3, was 72.7% of that of control subjects, increasing to 89.8% of mean weight of control subjects after refeeding (mean BMI 19.0 +/- 1.2). Mean BMI of control subjects was 21.6 +/- 2.7. Compared with the control group, patients' nitrogen was initially depleted by 24.5%, increased by 18.4%, but remained 10.6% below control values (P < 0.001). Body fat was depleted by 58.4%, increased by 89.7%, but remained 21.8% below control values (P < 0.001). Thus, despite a greater initial depletion and subsequently a greater net gain, body fat remained relatively more depleted after treatment than did nitrogen and protein. Anorexia nervosa patients were shown to readily replenish protein during nutritional rehabilitation.