The aim of this study was to demonstrate the prevalence of iodine deficiency among schoolchildren and the risk factors influencing it. One thousand five hundred and seventy-three schoolchildren were chosen from 14 schools in seven different regions in Istanbul. After all data relating to sociodemographic factors and the use of iodized salt were recorded, iodine contents of urine samples were determined by the Sandell Kolthoff reaction. Chi-squared and multiple regression analysis were used for the investigation of the correlation between iodine deficiency and risk factors. The prevalence of goitre (> or = 2 grade) was 1.9 per cent. The median urine iodine level was in the normal range (10.5 microg/dl). In 46.2 per cent of the students the urine iodine levels were below 10 microg/dl and 13.9 per cent of the students had urine iodine levels below 5 microg/dl. The prevalence of iodine deficiency was significantly higher in younger (< or = 10 years) children, in children with less educated mothers and fathers, and with poorer and more crowded families (p < 0.01, p < 0.01 and p < 0.01, p < 0.001, respectively). The rate of iodine deficiency was significantly higher in females than in males and also higher in children who lived on the European side of Istanbul than on the Anatolian side of Istanbul (p < 0.0001, p < 0.0001, respectively). The rate of use of iodized salt was 44.4 per cent and the iodine deficiency rate was not affected by the use of iodized salt (p > 0.05). Although the median urine iodine level was in the normal range, in 46.2 per cent of the students urine iodine levels were low (< 10 microg/dl). As there seems to be no effect from the use of iodized salt in iodine deficiency, the use of a more stable potassium iodate for the fortification of salt may be required.