Goto A, et al. - The true relationship between diabetes and cancer risk was investigated in this Mendelian randomization study including 10,536 subcohort subjects and 3,541 newly diagnosed cancer cases. For this case-cohort study, these participants were selected from 32,949 eligible participants aged 40-69 years within the Japan Public Health Center-based Prospective Study. Per doubling of the probability of diabetes, the estimated hazard ratios of cancer were 1.03 overall, 1.08 for the pancreas, 0.80 for the liver, and 0.90 for the colon. To support links between diabetes and overall and site-specific cancer risks, no strong evidence was found. The genetic role of type 2 diabetes in cancer development in a Japanese population was supported by little evidence.