We measured serum levels of estadiol (E2) and sex mn e-binding globulin (SHBG) among 50 healthy premenopausal Japanese women in 1994 in Gif Japan, to iiwestigate the reltionships between potential risk factor for brenst sncer and hormone levels. Using a self-adminis- No subject had a history of diabetes or endocrine diseases and none had ever taken oral contraceptives. There were no current users of other types of hormonal medication. One women had used hormonal medication about 2 years before the study, but only for a few months.Two blood samples were drawn from each subject in the mornings of the 11th and 22nd days of her menstrual cycle. Day 1 of the menstrual cycle was defined as the day of onset of menstrual bleeding. After centrifugation, the serum was separated and stored at -80°C until assayed. Hormone assays were carried out in one batch in one laboratory. Serum levels of total E2 and SHBG were measured radioimmunologically with kits purchased from Diagnostic Products Corporation (Los Angeles, CA). The intra-assay coefficients of variation, based on control pools, were 9.8% for E2 and 5.3% for SHBG.Hormone concentration was log-transformed for statistical analysis to reduce departures from the normal distribution. To assess the association between the logarithm of E2 or SHBG level and the study variables, we calculated the Pearson correlation coefficients. The variables examined in this way were height, weight, body mass index [BMI; weight (kg)/height (mi2)], age at menarche, age at start of regular cycle, birth order, and ages at menarche and delivery for the subject's mother. Although we restricted the sampling days according to the menstrual cycle, higher plasma E2 levels were observed for women with shorter cycles. Therefore, adjustments for cycle length and age were performed by regressing the logarithms of hormones and study variables separately on these potential confounders. The correlation coefficients between the two sets of residuals were then calculated. The relationships between categorical variables [i.e., length of the menstrual cycle (<27, 28-29, 30, 31-35 days, and irregular), smoking status, and