Toxifen, a nonsteroidal antiestrogen, is currently the most widely used adjuvant therapy for the treatment of breast cancer. Though the efficacy of tamoxifen has been well documented in clinical trials, the certainty over the duration of therapy is less clear. Clinical and laboratory evidence suggests that longer therapy is better; however, whether this means 5 years, 10 years, or indefinite therapy has not been established in clinical trials. Essential to any study of long-term tamoxifen therapy is consideration of its effects not only on breast cancer but also on other estrogen target tissues. The estrogenic effect of tamoxifen that lowers serum lipids results in fewer hospital admissions for heart disease and a reduction in fatal myocardial infarction. Similarly, tamoxifen maintains bone mass that may ultimately result in fewer fractures. The effects of tamoxifen appear to parallel the effects of estrogen, so results from clinical trials of estrogen replacement therapy will provide a useful guide of what to expect with tamoxifen. The negative aspect of the therapy is a modest increase in the incidence of endometrial cancer. However, the incidence of endometrial cancer after stopping either estrogen or tamoxifen remains elevated for at least 5 years after the drug is stopped. Nevertheless, it is important to stress that the overall prognosis remains unaffected. We conclude that it will be difficult to demonstrate survival differences between 5 and 10 years of tamoxifen in clinical trials unless significant tamoxifen-stimulated recurrences occur with continued therapy. The benefits of tamoxifen must be calculated using estimates of the decreased rates of heart disease, contralateral cancers, decreased hospitalizations for fractures, and reduced cancer relapses.