Evidence from epidemiology studies of the association of insulin-like growth factors with breast cancer risk has been rapidly accruing over the past few years, indicating a potential role of IGFs in breast carcinogenesis. However, several limitations that stem from both the size and design of the studies as well as from other methodological issues such as laboratory methods currently limit our ability to draw firm conclusions from published data. Nonetheless, bearing these shortcomings in mind, there is increasing evidence that an association exists between circulating IGF-I levels and risk of breast cancer, and that this relation differs by menopausal status. The prospective studies have been consistent in observing no association between circulating IGF-I levels and breast cancer risk among postmenopausal women. In contrast, four of five prospective studies have observed a relatively strong, positive association among premenopausal women. While findings from the retrospective case-control studies have not been consistent, likely due in part to the small size of many of the studies, the larger and more carefully conducted case-control studies have tended to observe similar associations of IGF-I levels with breast cancer risk. The recent reports of a correlation between IGF levels and mammographic density (a strong risk factor for breast cancer) in premenopausal women also provide indirect support for a relation with breast cancer. This review summarizes the evidence regarding IGF-I and breast cancer risk that has been generated through observational studies and discusses future research needs.