OBJECTIVES: The etiology of breast cancer is only partially understood. Based on the findings that pregnancies reduce breast cancer risk, a possible inverse association between exposure to the placental hormone human chorionic gonadotropin (hCG) and the risk of breast cancer has been suggested. Hydatidiform mole, a gestational trophoblastic disease, is associated with a high expression of hCG. We performed a population-based cohort study in which women with a history of hydatidiform mole were followed up for future cancer outcomes. METHODS: All 3371 women with a notification of hydatidiform mole in the Swedish Cancer Registry between 1958 and 1993 were followed up for future cancer outcomes by record linkages within the registry. RESULTS: In a total of 57,075 person-years of follow-up, 59 women had a diagnosis of breast cancer during follow-up, yielding an overall standardized incidence ratio of 1.3 (95% CI 1.0-1.7). CONCLUSION: This finding is not consistent with the hypothesis of a protective effect of hCG exposure on breast cancer risk, but rather suggests an adverse association.