Higher levels of insulin and insulin-like growth factor 1 (IGF-1) increase cancer risk by stimulating cell proliferation and increasing survival of DNA-damaged cells through antiapoptotic mechanisms. Laboratory studies suggest that flaxseed added to the diet may lower circulating levels of insulin and IGF-1, but there is limited information on the effects of dietary flaxseed on these biomarkers of cancer risk in humans. The aim of this study was to determine the effect of flaxseed supplementation in postmenopausal women on serum levels of insulin-like growth factor 1 (IGF-1), insulin-like growth factor binding protein 3 (IGF-BP3), and C-peptide, a marker of insulin production. Forty-eight postmenopausal women participated in this 12-wk baseline to postintervention study. Participants were asked to consume 7.5 g per day of ground flaxseed for 6 wk and 15 g per day for an additional 6 wk. No significant changes were observed in blood levels of IGF-1, IGF-BP3, or C-peptide over the study intervention. Flaxseed supplementation did not impact circulating levels of IGF-1, IGF-BP3, or C-peptide. Longer duration of intake may be necessary to observe changes in these biomarkers of cancer risk.