Tamoxifen is a nonsteroidal anti-estrogen frequently used in breast cancer therapy. Side effects to tamoxifen are uncommon (2%) but should be recognized and detected early by careful follow-up. Tamoxifen adjuvant therapy is absolutely indicated in postmenopausal breast cancer with estrogen-receptor--positive nodes. Recently, this indication has been extended to negative-node postmenopausal breast cancer. Mild acute side effects are the most frequent: hot flushes, menstrual irregularity, nausea, headache, vertigo, minimal modifications in blood cell counts. However, more serious accidents can occur. Increased risk of thromboembolism is linked to a fall in the level of antithrombin III. Ocular toxicity can occur. If such ocular lesions are diagnosed early enough, they can be cured by promptly withdrawing treatment. For patients given tamoxifen, there appears to be a small increase in risk of endometrial carcinoma, especially if the daily dose is > 30 mg. This over-risk requires adequate detection based on sufficient knowledge of the usual tamoxifen-related modifications in the endometrium. Physicians should also be aware of two favorable effects. Tamoxifen therapy leads to decreased cardiovascular morbidity and mortality in postmenopausal women and is associated with a significant increase in lumbar bone density. Risk of interaction with oral anticoagulants has been reported. We discuss here practical steps in the follow-up of women treated with tamoxifen.