Benign ovarian cysts have been suggested to influence breast cancer risk. To provide a comprehensive picture of the relation between ovarian cysts and breast cancer, we analyzed the data of 3 case-control studies conducted in northern Italy and the Swiss Canton of Vaud between 1983 and 2001. These studies included 6,315 incident, histologically confirmed breast cancer cases and 6,038 hospital-based controls. Odds ratios (OR) and the corresponding 95% confidence intervals (CI) were estimated using unconditional multiple logistic regression models, including terms for sociodemographic, menstrual and reproductive factors. Overall, 4.9% of breast cancer cases and 6.6% of controls reported a history of ovarian cysts, with a multivariate OR of 0.72 (95% CI 0.62-0.85). The inverse association between ovarian cysts and breast cancer was consistent in separate strata of age at menarche, parity, age at menopause status and family history of breast cancer. No meaningful differences were also found across strata of menstrual cycle length, oral contraceptive use, history of oophorectomy and body mass index. Thus, the inverse relation between ovarian cysts and breast cancer risk was not accounted for by earlier menopause, or by any difference in reproductive and menstrual characteristics. Although some hormonal correlates of ovarian cysts may have a role on breast cancer risk, a biological explanation of this inverse association is still unclear. [Ed.]