To support the ovarian hormone hypothesis in the etiology of breast cancer, a hospital-based case-control study with community controls was conducted to evaluate the relationship of intervals among menstrual and reproductive events to the risk of breast cancer in Korea. The cases were 190 breast cancer patients, who had been histologically diagnosed at Seoul National University Hospital from Jan. 1, 1993 to Jun. 30, 1994. Included were cancer-free women, who had undertaken the Gynecological examination at the same hospital (n=190). Women recruited for a survey of diabetes prevalence in Yonchon County, adjacent to Seoul City, were taken as a community control group (n=190). Information on menstrual and reproductive factors with other life-styles was collected through a direct interview by the well-trained interviewers. The adjusted odds ratios and the 95% confidence intervals were based on the unconditional logistic regression model. Likelihood ratio test for trend was applied for the ordinal variables. Early age at menarche, late age at natural menopause, late age at first full term pregnancy, and fewer number of full term pregnancies are independently associated with the high risk of breast cancer in Korea. Moreover, the interval between the age at menarche and the age at natural menopause of community controls (29.9±6.15 years) was significantly shortened compared to breast cancer cases (34.9±4.42 years). Particularly noteworthy was that intervals between the age at menarche and the age at first full term pregnancy of both control groups (9.0±3.72 years for hospital controls; 7.2±4.04 years for community controls) were significantly shortened compared to breast cancer cases (11.0±4.51 years). These findings support the hypothesis that the longer exposure to ovarian hormones during the reproductive years, the higher the risk of breast cancer.