Body composition was assessed by bioelectrical impedance analysis (BIA) in 11 female patients with Graves' disease and in 49 age-matched healthy Japanese women. Patients with Graves' disease were examined in the hyperthyroid state before treatment and in the stable euthyroid state after treatment with antithyroid drugs for 6 to 18 months. Body weight (BW), percent body fat (BF/BW), percent lean body mass (LBM/BW) and percent total body water (TBW/BW) were not statistically different between hyperthyroid Graves' patients and healthy subjects. Percent body cell mass (BCM/BW) was much lower in hyperthyroid Graves' patients than in healthy subjects (mean±SEM; 33.9± 2.4% vs. 41.5±0.5%, P<0.001). Percent ratio of extracellular water to total body water (ECW/TBW) was much greater in hyperthyroid Graves' patients than in healthy subjects (53.9±3.0% vs. 41.8±0.5%, P<0.001). These abnormal ratios, BCM/BW and ECW/TBW, were normalized after treatment. Serum free T₄ levels showed a positive correlation with ECW/TBW (r=0.779) and a reverse correlation with BCM/BW (r=-0.760) in all of the patients with Graves' disease examined. These findings indicate that body composition is affected by thyroid hormones and that body composition in hyperthyroid Graves' disease is characterized by decreased BCM associated with increased ECW.