Thyroid volume was measured by ultrasonography in 50 healthy adults (25 males, 25 females, age 20-70 years) living in the non-iodine deficient area of Amsterdam. Thyroid volume was 10.7 +/- 4.6 ml (mean +/- SD, range 2.7-20.4 ml). No relation was found between thyroid volume and any of the following parameters: plasma TSH, T4, T3, thyroglobulin (Tg), urinary iodine excretion, tobacco and alcohol consumption. Thyroid volume in males (12.7 +/- 4.4 ml) was greater than in females (8.7 +/- 3.9 ml, p = 0.0014), but no sex difference was observed in the ratio of thyroid volume to body weight (males, 0.16 +/- 0.05 ml/kg; females 0.13 +/- 0.06 ml/kg; NS). Thyroid volume was positively related to body weight, but not to age. We conclude that the sex difference in thyroid volume is due to the difference in body weight between males and females. Lean body mass is presumably the most important physiological determinant of thyroid size in subjects living in a non-iodine deficient area.