Genistein, an isoflavone in soybean products, has estrogenic activity and is used as a natural substitute for estrogen replacement therapy in postmenopausal women. Genistein was also shown to decrease fat pad weight in female mice. The primary objective of this study was to determine the effect of genistein on adipose tissue apoptosis in vitro and in vivo. 3T3-L1 preadipocytes and mature adipocytes were treated with 0, 1, 10, 100, and 400 micromol/L genistein and then assayed for apoptosis, whereas only mature adipocytes were assayed for viability. Mature adipocytes treated with genistein demonstrated a dose-related increase in apoptosis. Ovariectomized female mice (9 mo old) were given 0, 150, or 1,500 mg/kg genistein in the semipurified phytoestrogen-free casein-based diet for 3 wk (n=10). After mice were killed, body composition was determined by dual-energy X-ray absorptiometry analysis, and parametrial (PM), inguinal (ING), and retroperitoneal (RP) fat pads were weighed and assayed for apoptosis (% DNA fragmentation). Genistein (1500 mg/kg) reduced food intake (FI) by 14% (P<0.01) and body weight (BW) by 9% (P<0.01). Body composition was not significantly affected, but PM and ING weights were decreased 22% (P<0.05) and 19% (P<0.07), respectively, by 1,500 mg/kg genistein. Apoptosis in ING fat was increased 290% (P<0.05) by 1,500 mg/kg genistein. These findings show that oral genistein treatment can reduce BW, mobilize body fat, and induce apoptosis of adipose tissue in ovariectomized female mice. Thus, genistein may be useful in treating or preventing increased adiposity after menopause.