Breast cancer is either hormone independent or hormone dependent. Hormone-dependent tumors are responsive to hormonal manipulation about 80% of the time, because their cells contain specific steroid hormone receptors that bind to the hormone. Tumors lacking these receptors demonstrate response rates of less than 10%, and patients with such tumors are not candidates for hormonal therapy. Additive hormonal therapy, which lacks the morbidity and mortality of ablative procedures, is reversible, and does not require life-long replacement therapy, has replaced many of the ablative procedures. A recent, randomized, prospective, controlled clinical trial compared megestrol acetate and tamoxifen in 197 postmenopausal women with metastatic breast cancer. Efficacy, as measured by response rate and duration of remission, was equal in both groups. Moreover, each agent was associated with little toxicity. As women with stages I and II operable breast cancer have recurrences of their tumors after exposure to antiestrogen therapy, it is practical to consider treatment with megestrol acetate, whose mechanism of action of killing tumor cells may be different from that of tamoxifen.