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Background The radiation-related risk of breast cancer among women following the Chornobyl accident remains uncertain. During pregnancy, there is rapid cell proliferation in the breast while radioactive iodine from fallout exposure can concentrate in lactating breast tissues. Methods We conducted a standardized incidence ratio (SIR) analysis of breast cancer in a cohort of 2,631 women who were lactating and/or pregnant at any time during the two-month period of radioiodine fallout (April 26, 1986 - June 30, 1986). There were 37,151 person-years of follow-up, and 26 incident breast cancers were identified through linkage with the National Cancer Registry of Ukraine. Breast cancer rates among pregnant or lactating women were compared to the general population rates, and SIRs were adjusted for oblast, urban/rural, age, and calendar year. Results The SIR was non-significantly elevated for women lactating at the time of accident (SIR = 1.30, 95% CI: 0.40; 3.01), but not for women initiating lactation within the two months of the accident (SIR = 0.82, 95% CI: 0.32; 1.65) or women pregnant at the time of the accident (SIR = 0.75, 95% CI: 0.44; 1.18). Conclusions The increased SIR for breast cancer among lactating women is consistent with the results of a similar study in Belarus and indicates the need to quantify the radiation risk of breast cancer in a larger study of women lactating during the period of fallout exposure.