BACKGROUND: Iodine deficiency and endemic goiter have been reported in the past in The Netherlands, especially in the southeast. OBJECTIVE: To evaluate iodine intake and thyroid size in Dutch schoolchildren, contrasting those living in a formerly iodine-deficient region in the east (Doetinchem) with those living in an iodine-sufficient region in the west (Amsterdam area). DESIGN: Cross-sectional survey of 937 Dutch schoolchildren aged 6--18 years, of whom 390 lived in the eastern and 547 in the western part of the country. METHODS: Thyroid size was assessed by inspection and palpation as well as by ultrasound. Iodine intake was evaluated by questionnaires on dietary habits and by measurement of urinary iodine concentration. RESULTS: Eastern and western regions were similar with respect to median urinary iodine concentration (15.7 and 15.3 microg/dl, NS, Mann-Whitney U test), goiter prevalence by inspection and palpation (0.8 and 2.6%, P=0.08, chi-squared test), and thyroid volumes. The P97.5 values of thyroid volumes per age and body surface area group were all lower than the corresponding sex-specific normative WHO reference values. Iodized salt was not used by 45.7% of households. Daily bread consumption was five slices by boys and four slices by girls. Weekly milk consumption was 3 liters by boys and 2 liters by girls. Seafish was consumed once monthly. From these figures we calculated a mean daily iodine intake of 171 microg in boys and 143 microg in girls, in good agreement with the measured median urinary concentration of 16.7 microg/dl in boys and 14.5 microg/dl in girls. The sex difference in iodine excretion is fully accounted for by an extra daily consumption of one slice of bread (20 microg I) and one-seventh of a liter of milk (8.3 microg I) by boys. Thyroid volume increases with age, but a steep increase by 41% was observed in girls between 11 and 12 years, and by 55% in boys between 13 and 14 years, coinciding with peak height velocity. Girls have a larger thyroid volume at the ages of 12 and 13 ...