Caffeine intake has been proposed to influence breast cancer risk. Its effect may be mediated by hormonal changes. The relationships between caffeine-containing beverages (coffee, green tea, black tea, oolong tea, and cola) and serum concentrations of estradiol and sex hormone-binding globulin were evaluated in 50 premenopausal Japanese women. Intakes of caffeine and caffeine-containing beverages were assessed by a semiquantitative food-frequency questionnaire. Blood samples were obtained from each woman on Days 11 and 22 of her menstrual cycle. High intakes of caffeinated coffee, green tea, and total caffeine were commonly correlated with increasing sex hormone-binding globulin on Days 11 and 22 of the cycle after controlling for potential confounders [Spearman correlation coefficients (r) ranged from 0.23 to 0.31]. Green tea but not caffeinated coffee intake was inversely correlated with estradiol on Day 11 of the cycle (r = -0.32, p = 0.04). Although the effect of caffeine cannot be distinguished from effects of coffee and green tea, consumption of caffeine-containing beverages appeared to favorably alter hormone levels associated with the risk of developing breast cancer.