Weight loss and anorexia are significant complications of a variety of disorders and add morbidity to the underlying process. We observed marked weight gain (median, 5.1 kg; range, 0.9 to 20.1 kg) and appetite enhancement in 27 of the 28 patients with breast cancer receiving treatment consisting of high doses of oral megestrol acetate (480 to 1600 mg/d). Weight gain occurred regardless of pretreatment weight, extent of metastases, or response to therapy. Our results suggest a possible role for megestrol acetate in reversing anorexia and weight loss, thereby improving the quality of life of patients with cachexia. Further research is needed to establish the mechanism of weight gain and potential clinical applications.