Tamoxifen is the most used adjuvant drug in breast cancer treatment. Its main action is as an anti-oestrogen, but in the endometrium of some patients it acts as an oestrogen. Some investigators have even reported an increased risk of developing endometrial carcinoma. The question of how to follow-up these patients and how to identify patients at risk of developing endometrial premalignant changes was investigated by the noninvasive ultrasound method. The follow-up of 292 patients from before the start of adjuvant treatment with tamoxifen and 94 without tamoxifen treatment was conducted at regular intervals. The changes in endometrial thickness as measured by ultrasound and histopathological changes are reported. A thicker endometrium was found in patients with receptor positive breast cancer even before the treatment with tamoxifen started. Cumulative increasing thickness was found during treatment and this thicker endometrium remained until almost 3 years after the end of treatment. If the endometrium was <3 mm after 3 months of treatment the probability that it would be thin after 5 years was high. An increased risk of developing endometrial carcinoma was found, however due to this regular follow-up the cancer was identified at an early stage.