The preventive effects of physical activity against cancer may partly be ascribable to a possible decrease ininsulin-like growth factor-I (IGF-I) induced by the activity. To examine the association of physical activity withIGF-I and IGF-binding protein-3 (IGFBP-3), we analyzed the data for control participants in a case-controlstudy nested in the Japan Collaborative Cohort Study. A total of 3,598 men and 3,359 women throughout Japan,aged 40 to 79 years, were administered a lifestyle questionnaire and provided serum samples. The age- and bodymass index (BMI)- adjusted serum levels of IGF-I and IGFBP-3 were lower among those who walked for longertimes with a significant trend in both sexes (trend P < 0.01). Among participants who walked one hour or moreper day, the mean levels of serum IGF-I and IGFBP-3 were 138.7 ng/ml and 2.87 μg/ml in men and 125.7 ng/mland 3.14 μg/ml in women, respectively. The corresponding figures among those rarely walked were 147.4 ng/mland 2.99 μg/ml in men and 132.3 ng/ml and 3.21 μg/ml in women. For IGF-I, adjustment for serum IGFBP-3 didnot essentially alter such associations although the trend in women did not reach statistical significance. Adecreased level of serum IGF-I was associated with longer exercise time in men even after adjustment for serumIGFBP-3 (trend P = 0.033 after adjustment for age, BMI, and serum IGFBP-3), whereas the time was positivelycorrelated with serum IGF-I in women (trend P = 0.048). Our findings may partly explain the protective effectsof physical activity against several sites of cancer.