The objective of this study was to look at the effect of tamoxifen on the endometrium by comparing gynaecological cervical and endometrial cytology in breast cancer patients on tamoxifen for 3 years compared with controls. In addition, pelvic ultrasonography was employed to detect ovarian abnormalities and to measure endometrial thickness. Patients followed-up after primary surgical therapy for breast cancer were invited for gynaecological assessment consisting of clinical examination pelvic ultrasonography and a cervical smear and endometrial sampling. The patients taking tamoxifen (n = 49) has been on adjuvant hormone therapy for a minimum of 3 years. Control patients (n = 45) were also being followed-up for breast cancer. On examination the tamoxifen patients had very similar clinical findings to the control patients with regard to the cervix (normal in 84% of tamoxifen takers compared to 87% of controls). The uterus was clinically enlarged in eight patients on tamoxifen and in none of the control patients (P = 0.006) and only one ovarian cyst was clinically detectable in a patient taking tamoxifen. Pelvic ultrasonography between the two groups of patients was not statistically different (chi 2 test) and ovarian cysts were noted in nine patients from each group (tamoxifen patients 18% vs control patients 20%, n.s.). There was a highly significant difference in endometrial thickness in premenopausal patients (9.2 mm) compared with postmenopausal patients (6.4 mm, P = 0.001). There was also a suggestion that endometrial thickness was greater in tamoxifen treated patients (P = 0.08). In general, a greater proportion of patients taking tamoxifen had cervical and endometrial cells exhibiting hyperplastic nuclei, and in endocervical smears, this difference achieved statistical significance (Mann-Whitney test, P = 0.046). These findings show that a significantly increased proportion of patients taking tamoxifen had endocervical nuclear hyperplasia, and a trend towards increased endometrial thickness. These findings confirm that tamoxifen has mild oestrogenic activity. However, the lack of any difference in the incidence of dysplasia suggests that the carcinogenic potential of tamoxifen on the uterus is very low and the beneficial effects of tamoxifen as an adjuvant therapy for breast cancer outweighs its theoretical risks.