The nonsteroidal antiestrogen tamoxifen is the most widely used anticancer drug. In women with breast cancer, adjuvant therapy with tamoxifen reduces relapse and improves overall survival. In advanced breast cancer, the response rate is more than 50% in hormonal dependent disease. In women treated with adjuvant tamoxifen the incidence of new primary breast cancers is decreased. This latter observation has led to the initiation of prevention trials. In 1989 the first report from a large prospective randomised trial showed a significant increase of endometrial carcinoma among women treated with adjuvant tamoxifen. This effect may be linked to the somewhat paradoxical estrogenic properties of tamoxifen. The endometrial effects should be considered in the long term use of tamoxifen, and should also be taken into account in the evaluation of the prevention trials. Animal data indicate that tamoxifen can induce tumours in other organ systems, for example the liver, but no increase in primary liver cancer has been reported from the randomised trials. In some of these trials an increase in other gastrointestinal cancers (e.g. colon and gastric carcinoma) has been observed. The mechanism behind this may be different from that of the endometrium. In animal systems, tamoxifen has shown to induce DNA damage, with formation of DNA adducts. The risk of secondary gastrointestinal cancer needs to be further evaluated. The adverse effects of tamoxifen have led to the development of new anti-estrogenic drugs and other estrogen reducing agents (e.g. aromatase inhibitors).