The association between multiple births and subsequent maternal breast cancer risk was explored in a nested case-control study in Sweden encompassing 19,368 parous women with breast cancer diagnosed up to age 65 years, and 100,459 parous controls. Among cases and controls, there were 329 and 2,031 women, respectively, with a history of at least one live multiple birth. Compared with singleton mothers, breast cancer risk was 12 percent lower (odds ratio = 0.88, 95 percent confidence interval = 0.78-0.99) in women who had had a multiple birth. After stratification for age at diagnosis, evidence of a significant inverse association was found only in women aged 54 years or younger. Birth order of the multiple pregnancy had no apparent risk-modifying effect. Age at earliest multiple birth was unrelated to breast cancer risk. The inverse association between twinning and breast cancer risk may reflect protective physiological features of twin pregnancies. Further research is needed to investigate the role, if any, of increased levels of steroid hormone-binding globulins in mothers of twins and the proposed inhibitory effects of human chorionic gonadotropin and alpha-fetoprotein, both of which are increased during multiple gestations, on breast carcinogenesis. Breast feeding patterns in mothers of twins also may modify their risk of developing breast cancer.