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Pregnancy is a target period for events that could induce childhood leukemia. There has been little attention to possible effects of maternal occupational exposure to extremely low frequency magnetic fields (ELF-MF) during pregnancy.We conducted a population-based, case-control study of 491 incident cases of acute lymphoblastic leukemia in children 0-9 years of age, matched on age and sex to 491 healthy controls. Cases were diagnosed in the Province of Québec between 1980 and 1993. Mothers were interviewed to obtain detailed prenatal occupational history; individual exposure to ELF-MF was estimated based on a method we recently developed. We used 3 metrics for analyzing exposure: cumulative, average and maximum levels. Analyses were carried out among all study women and among working women only.Comparing the highest 10% of exposed mothers to the others, the risk of leukemia among offspring was moderately increased by using any metric, in all women and among working women only. The highest odds ratio of 2.5 (95% confidence interval = 1.2–5.0) was found for maximum exposure attained in an occupation (≥0.4 microtesla).Our results are compatible with an increased risk of childhood leukemia among children whose mothers were exposed to the highest occupational levels of ELF-MF during pregnancy.