OBJECTIVE This study aimed to clarify the iodine deficiency status in the Semipalatinsk region that has been contaminated by radioactive fallout from nuclear testing during the period of the former USSR. METHODS Based on the Japan-Kazakstan joint project of adult cancer screening around the Semipalatinsk Nuclear Testing Site (SNTS), from May to October 2002 spot urine specimens were collected at random in each village. Separately, children aged 5-15 years from around the SNTS were chosen at random and spot urine specimens were collected from them. METHODS Area contaminated by radioactive fallout around the SNTS, Republic of Kazakstan. METHODS A total of 2609 adults aged >40 years from 16 settlements in three regions and one city, and 298 children aged 5-15 years from two regions and one city. RESULTS Median urinary iodine concentrations of adults and children in all regions were in the range of 116.0-381.7 and 127.7-183.0 microg l(-1), respectively. The highest prevalence of values <50 microg l(-1) (14.1%) did not exceed 20%. Distributions within each group, adults and children, showed almost the same pattern, except for one region where more than 50% of adults had urinary iodine concentration >100 microg l(-1). CONCLUSIONS In agreement with our previous studies, the urinary iodine concentration data showed no clear evidence of iodine deficiency around the SNTS. Kazakstan is geographically and nutritionally at moderate risk of iodine deficiency disorders without fortification or iodine replacement by iodised salt. The socio-medical prophylaxis against iodine deficiency has been successfully maintained in East Kazakstan.