We reported previously that vitamin D deficiency is a causal mechanism of postoperative tetany in patients with Graves' disease. The aim of the present study was to determine the prevalence of vitamin D deficiency by reviewing serum 25(OH)D levels in 208 patients with Graves' disease (146 women, 62 men) during a 1 year period. Serum 25(OH)D levels were significantly lower (p<0.001) in female Graves' patients (31.8±13.3nmol/l) than in male patients (41.3±15.0nmol/l). Vitamin D deficiency (defined as a serum 25(OH)D value below 25nmol/l) was found in 40% of female patients and in 18% of male patients (p<0.005). There was a significant seasonal variation in the 25(OH)D concentrations in female patients [amplitude 6.38 (95% CI, 5.42-7.56)], with values below 25nmol/l found in 58% of female patients during the winter months. There were significant (p<0.001) differences in serum 25(OH)D levels between age groups in the female patients. The concentrations were lowest in patients in their twenties (25.1±8.2nmol/l) and highest in patients in their fifties and sixties (43.2±13.7nmol/l). Serum 25(OH)D concentrations might be monitored in patients with Graves' disease during antithyroid drug therapy, and vitamin D and/or calcium supplements are recommended for patients with vitamin D deficiency.