Self-reported type 2 diabetes mellitus (T2DM) is a risk factor for many cancers, suggesting its pathology relates to carcinogenesis. We conducted a case-cohort study to examine associations of hemoglobin A1c (HbA1c) and c-peptide with cancers associated with self-reported T2DM. This study was drawn from a prospective cohort of 32,383 women and men who provided blood specimens at baseline: c-peptide and HbA1c were assessed in 3,000 randomly selected participants who were cancer-free-at-baseline and an additional 2,281 participants who were cancer-free-at-baseline and subsequently diagnosed with incident colorectal, liver, pancreatic, female breast, endometrial, ovarian, bladder, or kidney cancers. Weighted Cox regression models estimated HRs and 95% confidence intervals (CI), adjusted for covariates. c-peptide was associated with higher risk of liver cancer [per SD HR: 1.80; 95% CI: 1.32–2.46]. HbA1c was associated with higher risk of pancreatic cancer (per SD HR: 1.21; 95% CI: 1.05–1.40) and with some suggestion of higher risks for all-cancers-of-interest (per SD HR: 1.05; 95% CI: 0.99–1.11) and colorectal (per SD HR: 1.09; 95% CI: 0.98–1.20), ovarian (per SD HR: 1.18; 95% CI: 0.96–1.45) and bladder (per SD HR: 1.08; 95% CI: 0.96–1.21) cancers. Compared with no self-reported T2DM and HbA1c < 6.5% (reference group), self-reported T2DM and HbA1c < 6.5% (i.e., T2DM in good glycemic control) was not associated with risk of colorectal cancer, whereas it was associated with higher risks of all-cancers-of-interest combined (HR: 1.28; 95% CI: 1.01–1.62), especially for breast and endometrial cancers. Additional large, prospective studies are needed to further explore the roles of hyperglycemia, hyperinsulinemia, and related metabolic traits with T2DM-associated cancers to better understand the mechanisms underlying the self-reported T2DM-cancer association and to identify persons at higher cancer risk.Significance:The results from this study suggest that HbA1c and c-peptide, markers of hyperglycemia and hyperinsulinemia respectively, are associated with certain cancers, though people with diabetes may be at increased risk of these cancers, perhaps other than colorectal, even when their glucose is well controlled.