Nearly 15 years ago, female carriers of the fragile X mental retardation syndrome were noted to have an increased incidence of twin pregnancies. Since then, much evidence has accumulated supporting the notion of ovarian dysfunction in fragile X carriers, in the forms of increased dizygotic twinning and premature ovarian failure. However, despite a decade and a half of research regarding this association, the underlying mechanism remains a mystery. This article reviews the population-based studies that have examined this association and discusses possible reasons for the variations in results. In addition, results from more recent studies on endocrine function in fragile X carriers are discussed. These data, when considered in conjunction with our emerging understanding of the molecular biology of the fragile X gene (FMR1) and its protein product (FMRP), are beginning to elucidate possible mechanisms for the association between fragile X syndrome and ovarian dysfunction.