There is evidence that the insulin-like growth factor system (IGF), particularly IGF-I, is important in human carcinogenesis. We studied in a general, though not strictly random population sample of 620 adults, the relationship of IGF-I to demographic, lifestyle and dietary factors, the latter ascertained through an extensive validated questionnaire. Plasma IGF-I levels declined significantly with age and the decline was more evident among women than among men. Tobacco smoking, body mass index and regular physical activity were unrelated to this hormone and a positive association with height was not statistically significant. Neither protein nor carbohydrate intake was related to plasma IGF-I levels but there was inconsistent evidence that ethanol intake may be inversely associated with plasma IGF-I and saturated and polyunsaturated lipids may be positively associated with it. The findings are evaluated in conjunction with evidence indicating that the incidence of cancer is lower among women than among men, height is a risk factor for several forms of cancer, and saturated and polyunsaturated lipids have been more closely linked to human and animal carcinogenesis than monounsaturated lipids.