Adjuvant antiestrogen therapy for breast cancer. Past, present, and future. Laboratory investigations using animal models of breast cancer growth have indicated that the antiestrogenic compound tamoxifen is a tumoristatic agent. It is therefore effective in suppressing, rather than destroying, the breast tumor. Its use as an adjuvant in breast cancer management has been successful, with a proportion of women benefiting from long periods of tamoxifen treatment. All the initial studies recruited postmenopausal women, but tamoxifen is now proposed for the treatment of premenopausal women for an extended time. Naturally, there are many aspects of the toxicology of tamoxifen to consider; however, careful monitoring of clinical trials will determine the safety of the drug for the general patient population. 