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Objective: An earlier investigation reported a five-fold increase in breast cancer risk among women who smoked during pregnancy. Using a similar design, we re-examined this hypothesis. Methods: The source population comprised Massachusetts residents who gave birth between 1987 and 1999 with a birth record in the Massachusetts Vital Statistics Registry. Cases were diagnosed with breast cancer between 1988 and 2000 at ages 25-55 with a record in the Massachusetts Cancer Registry. Three controls were matched to each case on maternal age, year of giving birth, and birth facility. Information on smoking, the matched factors, and potential confounders were collected from the birth certificate. The data were analyzed using conditional logistic regression. Results: After adjusting for potential confounders, women who smoked during pregnancy did not have an increased risk of breast cancer compared to women who did not smoke during pregnancy (relative risk = 1.0, 95% Confidence interval CI = 0.81-1.2). We observed no dose response relation between number of cigarettes smoked per day during pregnancy and breast cancer risk. There was no evidence that our results were biased by misclassification from women inaccurately reporting their smoking status. Conclusion: In contrast to the previous study, we did not observe an increased risk of breast cancer in women who smoked during pregnancy.