OBJECTIVE The aim of the study was to investigate the endometrial histology and the bleeding pattern under a hormone replacement therapy regimen with continuous estrogen quarterly (3-monthly) combined with a progestogen. METHODS In a prospective, double-blind, randomised clinical trial, 30 healthy, postmenopausal women were allocated to one of the three trial preparations. Group I was treated with 1 mg micronized 17 beta-estradiol continuously, group II took 2 mg micronized 17 beta-estradiol continuously and group III took 1 mg and 2 mg 17 beta-estradiol alternating every 42 days (step-up regimen). One treatment cycle was 84 days, during the last 12 days estradiol was combined with 50 micrograms gestodene. The total treatment period comprised two cycles of 12 weeks each. With regard to endometrial histology, the second cycle was the actual study cycle. In each patient endometrial samples were obtained at the following time points: after the withdrawal bleeding in the beginning (day 8-11) of cycle II (Vabra-method), at the end of the estrogen mono-phase (day 70-72) of cycle II(Pipelle-method), and 8-11 days after cessation of all medication (Vabra-method). Histopathological classification was done by two experienced gynaecological pathologists. All patients kept record of their bleeding events in a diary. Analysis of variance and Kruskal-Wallis test were used for statistical analysis of the data. RESULTS 29 patients were evaluable for the assessment of endometrial histology. Only one sampling procedure (1.2%) yielded an insufficient amount of tissue. In each treatment group, simple (cystic) hyperplasia was observed exclusively at the end of the estrogen mono-phase (in total 4/29 patients, 14.8%). Hyperplasia disappeared in all cases after the combined estrogen-progestogen phase. No cytological atypia was seen. Fifty-five cycles were evaluable for the bleeding pattern. The onset of the scheduled bleeding (withdrawal bleeding) was in all cycles on day 11 of the combined phase or beyond. Unlike the duration, the severity of the scheduled bleeding episodes was estrogen-dose dependent. In the entire treatment period of 2 x 84 days, breakthrough bleeding occurred in 3 women, totalling 9 days. Spotting occurred rarely and was equally divided among the treatment groups. CONCLUSIONS A quarterly sequential hormone replacement therapy regimen for women with anintact uterus gives rise to the development of simple hyperplasia without cytological atypia at the end of the unopposed estrogen phase. This occurs independent of the estrogen dose and can be reverted to inactive or atrophic endometrium by the addition of gestodene during 12 days. The combination offers good cycle control. The safety aspects should be investigated further in long-term studies before this regimen can be advocated for routine use.