We used transvaginal color Doppler sonography to study the effects of hormone replacement therapy on the endometrial structure and vascular flow of 345 normal postmenopausal women. We studied women taking estrogen replacement alone, continuous combined estrogen and progestogen, and estrogen followed sequentially by estrogen-progestogen combination. Endometrial measurements prior to the initiation of hormone replacement therapy were used as baseline values. An increase in endometrial thickness was seen soon after initiation of hormone replacement therapy (P < 0.0001). Hyperplasia or adenocarcinoma was found only when endometrial thickness was greater then 9 mm. No correlation was found between hormone replacement therapy and the occurrence of endometrial hyperplasia or adenocarcinoma.