The effect of tamoxifen on the vaginal mucosa and on serum oestrone, oestradiol and gonadotrophin concentrations was investigated in a group of nine postmenopausal women with non-metastatic breast cancer. Compared with fit age-matched controls, the vaginal pH in the tamoxifen-treated group was significantly lower and was comparable to levels in fertile women. Two thirds of tamoxifen-treated women had well oestrogenized vaginal smears compared with none in the control group. Follicle-stimulating hormone was significantly reduced, whilst oestrone and oestradiol levels remained unchanged. We conclude, therefore, that tamoxifen has oestrogen-agonistic properties particularly evident in postmenopausal women, even though it is primarily an antioestrogenic drug.