Thirty post-menopausal, non-hysterectomised women received a 50 mg oestradiol implant subcutaneously and either 10 mg or 20 mg dydrogesterone daily for 14 days every 28 days for 6 months. Endometrial biopsies were taken during the initial oestrogen-only phase and again during the final progestogen phase. Of the ten initial samples which were adequate for histological diagnosis, nine showed proliferative and one non-secretory endometrium. Of the 28 samples obtained at the end of the study during the progestogen phase, all except one showed satisfactory conversion, irrespective of dose. Acceptable bleeding patterns were seen in both dosage groups. Tolerance was good and no patient discontinued treatment. This study has shown that both 10 mg and 20 mg dydrogesterone for 14 days are potent enough to oppose the proliferative effects of the 50 mg oestradiol implant. In view of the wide inter-patient variation in endometrial response to progestogens, it appears appropriate to choose the dosage of dydrogesterone on the basis of cycle control and tolerability, whilst being able to maintain confidence in endometrial protection.