Thymulin (or FTS-Zn) a well-defined thymic hormone was studied in fifteen female patients hospitalized for anorexia nervosa. The circulating hormone was measured together with the plasma levels of thyroid hormones, cortisol and zinc. Thymulin activity determined by the rosette assay was significantly reduced in the anorexia nervosa patients compared to sex- and age-matched healthy control subjects. The patients were characterized by very depressed plasma levels of triiodothyronine (T3) but exhibited normal concentrations of thyroxine (T4), thyroxine-binding globulin (TBG), cortisol and zinc. The distribution of their peripheral lymphocyte cells into several subsets was not affected. The observed decrease of thymulin activity in this illness might be the consequence of thymic atrophy secondary to malnutrition and/or hormonal disturbances. Our results suggested that the fall in thymulin level might explain the variability of cellular immune responses in anorexia nervosa patients and occurrence of energy when their weight loss is far advanced.