Genistein is a naturally occurring isoflavone phytoestrogen commonly found in plant products such as soybeans, lentils, and chickpeas. Genistein, like other phytoestrogens, has the potential to mimic, enhance, or impair the estradiol biosynthesis pathway, thereby potentially altering ovarian follicle growth. Previous studies have inconsistently indicated that genistein exposure may alter granulosa cell proliferation and hormone production, but no studies have examined the effects of genistein on intact antral follicles. Thus, this study was designed to test the hypothesis that genistein exposure inhibits follicle growth and steroidogenesis in intact antral follicles. To test this hypothesis, antral follicles isolated from CD-1 mice were cultured with vehicle (dimethyl sulfoxide; DMSO) or genistein (6.0 and 36 μM) for 18–96 h. Every 24 h, follicle diameters were measured to assess growth. At the end of each culture period, the media were pooled to measure hormone levels, and the cultured follicles were collected to measure expression of cell cycle regulators and steroidogenic enzymes. The results indicate that genistein (36 μM) inhibits growth of mouse antral follicles. Additionally, genistein (6.0 and 36 μM) increases progesterone, testosterone, and dehydroepiandrosterone (DHEA) levels, but decreases estrone and estradiol levels. The results also indicate that genistein alters the expression of steroidogenic enzymes at 24, 72 and 96 h, and the expression of cell cycle regulators at 18 h. These data indicate that genistein exposure inhibits antral follicle growth by inhibiting the cell cycle, alters sex steroid hormone levels, and dysregulates steroidogenic enzymes in cultured mouse antral follicles.