At the end of the winter circulating concentrations of 25-hydroxyvitamin D (25-OHD) in Asian and Mediterranean immigrants in northwestern European countries are usually very low. This may lead to vitamin D deficiency and eventually to rickets. Children are more prone to develop vitamin D deficiency. The vitamin D status of 8-year-old Turkish, Moroccan and Caucasian children was assessed by measuring plasma concentrations of 25-OHD and parathyroid hormone (PTH) and related to the cumulated global sun radiation (CGSR). The study population was selected from school children in The Hague and Rotterdam (The Netherlands). In each city blood samples were examined from 40 migrant children and a Caucasian reference group (n = 40) (matched for age and sex). Children in The Hague were examined at the end of the winter and early spring (February/April) and those in Rotterdam in late spring and early summer (May/June). Mean plasma 25-OHD concentrations for the migrant children in both cities were significantly lower than for the Caucasian reference group. The lowest 25-OHD values were measured in migrant children in The Hague: 42 per cent of the Turkish and 23 per cent of the Moroccan children had plasma levels less than 20 nmol/l. In these groups hyperparathyroidism (PTH greater than 85 pmol/l) was significantly more prevalent. With all pooled data a significant correlation between plasma 25-OHD and PTH was observed (r = -0.24; P less than 0.001). Regression analysis indicated that the relationship between both variables did not differ among the three ethnic groups. As expected, 25-OHD concentrations were higher for increased CGSR. Differences in 25-OHD level are mainly explained by differences in skin pigmentation and calcium intake between Mediterranean and Caucasian children. The secondary hyperparathyroidism observed in some children confirms that migrant children have a higher risk of developing nutritional rickets when living in areas with a moderate climate.