This case-control study was conducted to estimate the radiation-induced risk of acute leukemia during the period from 1987 to 1997 among residents 0 to 5 years of age at the time of the Chernobyl accident in the most radioactively contaminated territories of the Ukraine (Rivno, Zhytomyr, Chernihiv, and Cherkasy regions). Data were collected from 246 leukemia cases diagnosed between 1 January, 1987, and 31 December, 1997. Each case was verified and interviewed. Verified cases were compared to 492 randomly selected controls matched by age, sex, type of settlement (rural, semirural, urban), and administrative region of residency. The cumulative level of radiation exposure from the time of the Chernobyl accident to the date of diagnosis was assessed for each case and corresponding controls. Four dose-range groups were selected for statistical analysis (0-2.9 mGy, 3-9.9 mGy, 10-99.9 mGy, and 100 - 313.3 mGy).The risk of leukemia was significantly increased (-2.4 [95%CI: 1.4-4.0]) among those with radiation exposure doses higher than 10 mGy (p=0.01). The association between radiation exposure and risk was stronger among males (-2.8 [95%CI: 1.4-5.5, p=0.01]), and for cases of acute leukemia that were diagnosed during the period from 1987 to 1992 (-2.5 [95%CI: 1.2-5.1, p=0.05]), particularly acute myeloid leukemia (-5.8 [95%CI: 1.4-24.6, p=0.05]). The influence of possible confounders and methods of selecting controls on the leukemia risk assessment was analyzed. The evaluated risk per unit dose is discussed. (c) 2009 UICC.