This study was developed to investigate leukaemia risk among workers potentially exposed to external radiation during clean-up operations following the Chornobyl accident. The study includes a cohort of around 110,000 male Ukrainian clean-up workers who worked in the 30km zone around the plant from 1986 to 1990. Eligible workers were identified through the State Chornobyl Registry. Potential cases of leukaemia between 1986 and 2000 were identified using record linkage and used to create a Provisional Leukaemia Registry. For each case, five controls were drawn from cohort members who did not develop hematologic disorders, matched to the case on age and area of residence.