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Early studies of breast cancer raised substantial concern regarding risk associated with induced abortion and miscarriage. Literature reviews suggest that study findings depend heavily on the comparison group and that the use of parous women as a reference group for nulliparous women may artificially inflate risk. To examine the individual effects of induced abortion and miscarriage on breast cancer risk of parous and nulliparous women, 744 patients < or =40 years of age and diagnosed from 1983-1988 were matched by parity, age, and race with controls living in the same neighborhood in Los Angeles County. In-person interviews were conducted to obtain a detailed reproductive history. Risk estimates were obtained by conditional logistic regression using nulligravid women as the reference group for nulliparous women with a history of incomplete pregnancy and parous women with no incomplete pregnancies as the reference group for parous women with a history of incomplete pregnancy. Breast cancer risk of parous women was unrelated to a history of miscarriage or induced abortion. Breast cancer risk was reduced among nulliparous women with a history of induced abortion relative to nulligravid women, although the risk estimate was imprecise. Risk declined as the number of induced abortions increased (P = 0.04). Our results do not support the hypothesis that induced abortion or miscarriage increase the breast cancer risk of young women.