Variation in the circulating concentrations of the insulin-like growth factor (IGF) system has been implicated in the etiology of chronic diseases including cancer (prostate, breast, colon, and lung), heart disease, type 2 diabetes, and osteoporosis. We searched for sociodemographic, anthropometric, reproductive, lifestyle, and dietary determinants of IGF-I and insulin-like growth factor binding protein (IGFBP) -3 serum concentrations. Serum samples were collected in a Singapore Chinese cohort with a mean age of 61 years. Subject information was assessed during an in-person interview. Radioimmunometrically measured IGF-I and IGFBP-3 concentrations were available for 312 men and 326 postmenopausal women ages 50 years or older. Mean IGF-I concentrations were 144 ng/ml and 121 ng/ml for men and women, respectively (gender difference, P < 0.0001), and mean IGFBP-3 concentrations were 3710 ng/ml and 4147 ng/ml for men and women, respectively (gender difference, P < 0.0001). IGF-I and IGFBP-3 decreased with age (P for trend <0.0001); the age-related decrease in the IGF-I:IGFBP-3 molar ratio was stronger in women than men. IGF-I concentrations were higher among physically inactive subjects and among women with an early age at menarche. Consumption of saturated fat was found to decrease, and intake of omega-3 polyunsaturated fatty acids and of dietary fiber was found to increase circulating IGFBP-3 concentrations. Intake of calcium from food and supplement was associated positively with circulating IGF-I, IGFBP-3, and molar ratio. Intake of soy was associated positively with IGF-I and molar ratio concentrations, but only in men. The results of this study lend additional support to the hypothesis that circulating IGF-I concentrations increase the risk of prostate, bladder, colorectal, and breast cancer.