The plasma concentrations of oestrone, oestradiol-17β and progesterone, and the urinary excretion of oestrone, oestradiol-17β, oestriol and pregnanediol have been determined in normal Japanese and British adolescent, pre-menopausal, menopausal and post-menopausal women. The amounts of plasma oestrogen in British and Japanese women were not significantly different except in the adolescent group. In this latter category the mean level of plasma oestrone and oestradiol-17β in the luteal phase was significantly higher in Japanese compared with British women. The mean excretion of urinary oestrogens in the two races was similar for all age groups studied, as was plasma progesterone and urinary pregnanediol. The mean ratio of oestriol to oestrone and oestradiol-17β for Japanese was higher than that for British women in the two pre-menopausal groups, although the difference was not statistically significant. The difference in the incidence rates of breast cancer in Britain and Japan does not appear to be related to differences in plasma levels of oestrogens or progesterone.