The incidence of breast cancer is increasing rapidly in Taiwan but is still low compared with western countries. The impact of body size factors in western countries inspired us to have a case-control study. A matched case-control study was done on 122 pairs of incident cases of female breast cancer and community controls. Body height and weight were collected from a questionnaire interview and used to derive the body mass index (kg/m2). Demographic and reproductive characteristics were collected and controlled as potential confounders. Conditional multiple logistic regression analysis was used to estimate the multivariate-adjusted odds ratios for each risk factor regarding body size. After adjustment for age, schooling years, age at menarche and parity; body weight and height were positively related to postmenopausal breast cancer but not significantly related to the risk of premenopausal breast cancer. The odds ratios for body mass index were less prominent. Women with a higher breast stem cell mass resulting from better adolescent nutrition had a higher risk for breast cancer; and postmenopausal obesity may increase the breast cancer risk through the increased conversion of adrenal androstenedione to estrogen in adipose tissue after menopause.