Iodine deficiency is the major cause of an increase in thyroid gland volume in infants and children. In this field study we monitored the iodine supply and its effect on the thyroid gland volume in prepubertal and pubertal children in the eastern and western parts of the city of Berlin, so far considered as an area with borderline iodine deficiency. The thyroid gland volume was determined by ultrasound in 1080 (f = 552, m = 528) children aged 3-15 years, and was correlated to age, body-surface area and iodine excretion, which was measured in a first-morning spot urine. The mean iodine concentration was 115.8 micrograms iodine/g creatinine (12.2 micrograms iodine/dl urine), with no significant differences between eastern parts with 114.5 micrograms iodine/g creatinine (12.3 micrograms iodine/dl urine) vs 116.7 micrograms iodine/g creatinine (12.0 micrograms iodine/dl urine) in the western parts of the city. This good iodine supply of the children was surprising compared to former studies in children and adults. Moreover this normalization of the iodine excretion was reflected by smaller thyroid gland volumes in the children. The volume was found to increase with age and was 2.4 +/- 1.1 ml in prepubertal (Prader and Largo: f < or = 10.9 ys, m < or = 11.5 ys) children, compared to 4.3 +/- 1.7 ml in pubertal children. The goiter prevalence, calculated on this data was below 5%. Among all children there were only 11 (aged 8-13 ys) with abnormal findings of the thyroid gland on ultrasound: 6 with small nodules, 1 girl with a thyroid-cyst, 2 girls had an inhomogenous echo structure and 2 girls presented with a hemithyroidea. This study shows that the iodine supply of the children in Berlin has improved, resulting in smaller sized thyroid glands, compared to those which have been previously published for Germany (Müller-Leisse 1988; Klingmüller, 1991; Menken, 1992), but they correspond well to volumes described in countries with sufficient iodine supply.