Growing evidence substantiates the role of the insulin-like growth factor I (IGF-I) system in breast tumorigenesis. Retinoids have been shown to affect the IGF system in several experimental models. We extended our previous data on plasma IGF-I modulation by the synthetic retinoid fenretinide (4-HPR) and investigated the effect of the retinoid on plasma IGF binding protein (BP)-3, the major protein binding IGFs. IGF-I and IGFBP-3 were measured on plasma samples obtained at randomization and after an interval of approximately 1 year, from 39 and 33 stage I breast cancer patients assigned to receive 4-HPR, and from 39 and 34 untreated controls, respectively. There was a significant decrease in plasma IGF-I after 4-HPR administration, whereas no significant change was observed in controls. The effect of 4-HPR on IGF-I levels was modified by menopausal status, inasmuch as the decrease in IGF-I was particularly pronounced in premenopausal women, whereas the reverse was observed in untreated controls. By contrast, treatment induced an increase of IGFBP-3 with respect to controls. As a result of this dual effect, the bioavailability of IGF-I for interaction with receptors at target levels further decreased in pre-menopausal 4-HPR-treated patients compared with controls, suggesting that retinoid administration may result in lower concentrations of biologically active IGF-I. Our findings may have important implications for the clinical preventive activity of this retinoid.