Foxo transcription factors have been implicated in diverse biological processes, including metabolism, cellular stress responses, and aging. Here, we show that Foxo3a^–/– female mice exhibit a distinctive ovarian phenotype of global follicular activation leading to oocyte death, early depletion of functional ovarian follicles, and secondary infertility. Foxo3a thus functions at the earliest stages of follicular growth as a suppressor of follicular activation. In addition to providing a molecular entry point for studying the regulation of follicular growth, these results raise the possibility that accelerated follicular initiation plays a role in premature ovarian failure, a common cause of infertility and premature aging in women.