The purpose of this work was to determine by ultrasound the volume and echo structure of the thyroid gland in 13-year-old schoolchildren in northern Finland. 76 healthy schoolchildren underwent cervical ultrasound examinations during the period of Jan-Feb 1990, performed with a real-time scanner using a 7.5 MHz linear transducer and direct contact method. The volume of each lobe was calculated according to the formula for a volume of rotation ellipsoid by multiplication of maximal thickness, width and height of the lobe by the correction factor 0.479. Any focal lesion that could be distinguished in the homogenous thyroid parenchyma was assessed for echogenicity as compared with the normal thyroid gland and was measured with electronic calipers. The mean thyroid volume was 6.5±1.6 ml (mean ±sd), being 6.3±1.8 ml in the boys and 6.7± 1.4 ml in the girls. The sex difference was not significant. The right lobe was significantly larger than the left one (mean 3.7 and 2.8 ml, respectively). Thyroid volume was correlated with body weight and body surface area in both sexes. Abnormal lesions in terms of echo structure were noted in one subject (1.3%). Comparing the results with the findings reported from other countries, it can be concluded that the thyroid volume in these 13-year-old Finnish schoolchildren was about 30% less than that reported for the same age group in the FRG (with insufficient iodine intake) and about 35% more than in 13-year-old schoolchildren in Sweden (with sufficient iodine intake).