Evaluation of endometrial thickness by transvaginal ultrasonography (TVUS) in postmenopausal estrogen receptor positive breast cancer patients treated with anastrozole after tamoxifen therapy. This study included 70 postmenopausal estrogen receptor positive breast cancer patients who switched to anastrozole after tamoxifen; patients had endometrial thickness >4mm and no endometrial malignancy. Endometrial thickness was measured after anastrozole treatment. Endometrial thickness during anastrozole therapy was lower than after tamoxifen therapy (p<0.001); the mean reduction in endometrial thickness was 4.5mm (+/-3.0). Cystic endometrial appearance was more frequent in patients under tamoxifen than in those under anastrozole (p<0.001). Duration of tamoxifen therapy was not correlated to the endometrial thickness at the time of its suspension. Duration of tamoxifen therapy and endometrial thickness at the time of tamoxifen suspension was correlated to the relative reduction of endometrial thickness during anastrozole therapy. Anastrozole reverses tamoxifen-induced increased endometrial thickness and sonographic endometrial cystic appearance.