NCI conducted a study of the incidence of leukemia and other cancers among Chernobyl clean-up workers from Estonia and Latvia who were followed from 1986 to 1998. Investigators did not observe a significantly elevated risk of leukemia or of all cancers combined, but thyroid and brain cancer were increased. There was no evidence of dose-response for any of these sites, and the relationship to radiation exposure remains to be determined. Further follow-up of the Chernobyl clean-up workers, led by collaborators in Estonia, is underway.