Hyperinsulinemia, high insulin-like growth factor 1 (IGF1) levels, and low IGF binding protein 1 (IGFBP1) levels have been implicated in the relationship between obesity and increased risk of colorectal cancer (CRC). However, it remains inconclusive whether circulating biomarkers of insulin and the IGF axis are associated with conventional adenoma and serrated polyp, the two distinct groups of CRC precursors.We prospectively examined the associations of plasma C-peptide, IGF1, IGFBP1, IGFBP3, and IGF1 to IGFBP3 ratio with conventional adenoma and serrated polyp among 11 072 women from the Nurses' Health Studies. Multivariable logistic regression was used to calculate the odds ratio (OR) per 1-SD increase in each biomarker for overall risk of conventional adenoma and serrated polyp and according to polyp feature.During 20 years of follow-up, we documented 1234 conventional adenomas and 914 serrated polyps. After adjusting for various lifestyle factors (including body mass index), higher concentrations of IGFBP1 were associated with lower risk of serrated polyp (OR = 0.84, 95% confidence interval = 0.75 to 0.95, =.005). The association was particularly strong for large serrated polyp (≥10 mm) located in the distal colon and rectum (OR = 0.59, 95% confidence interval = 0.39 to 0.87, =.01). In contrast, we did not find any statistically significant association between the biomarkers and conventional adenoma.A higher plasma level of IGFBP1 was associated with lower risk of serrated polyp. Our findings support a potential role of IGFBP1 in the serrated pathway of CRC in women.