NCI and collaborators are studying leukemia risk among Chernobyl clean-up workers in Ukraine. A nested case-control study has been carried out in a cohort of approximately 110,000 clean-up workers, who were in the 70-kilometer zone around the plant during 1986 to 1990. Leukemias arising between 1986 and 2000 were sought in all relevant health care institutions and were linked to cohort members. To estimate individual bone marrow doses, detailed dosimetry interviews were conducted using a method devised by a group of international specialists. A series of papers describing the study methods and results were published in 2008. The study was extended to accrue an additional 6 years of cases and has been completed, bringing the period of follow-up to 20 years. The radiation-related risk for leukemia excluding chronic lymphocytic leukemia (CLL) in the workers was comparable to that from the atom-bomb survivors. Investigators also observed a dose-response for CLL, which they plan to explore further by examining the relationship of monoclonal lymphocytosis (putative precursor of chronic lymphocytic leukemia) with radiation exposure among the cleanup workers.