The volume of the thyroid gland was determined by ultrasonography in 256 euthyroid subjects aged 0-20 years in Brussels, an area with borderline iodine intake (median urinary iodine: 6.8 micrograms/dl). The volume of each lobe was calculated separately using the formula of an ovoid (Depth x Length x Width x pi/6). The total thyroid volume was obtained by summation of the volume of both lobes. In neonates, mean volume (SD) was 0.84 (0.38) ml and the distribution was asymmetric, skewed towards elevated values (median: 0.76 ml); the volume was best correlated with body surface area (P less than 0.01). Thyroid volume significantly increased (P less than 0.001) until the age of 8 without being influenced by sex and thereafter varied widely: it increased from 2.7 (0.8) ml in prepubertal subjects aged 8-11 years to 11.6 (4.4) ml in late pubertal aged greater than 17 years. This increase was significantly correlated not only with chronological age but also with pubertal stage and seemed to happen early, with the onset of the first clinical signs of puberty. At all ages, the volume of the right lobe was slightly higher than the left lobe but the difference was not significant.