Diabetes and cancer are common diseases both with enormous impact on health burden globally. The increased risk of several types of cancer among people with type 2 diabetes mellitus has been indicated repeatedly. This study aimed at exploring and describing the association between type 2 diabetes and cancer incidence. A cohort of 428,326 people with type 2 diabetes was identified from the Finnish National Diabetes Register and followed up through a register linkage with the Finnish Cancer Registry for cancer incidence during 1988–2014. A total of 74,063 cases of cancer occurred in this cohort in 4.48 million person-years. This accounted for 16% more than the expected cancer incidence in the Finnish general population; the standardized incidence ratio (SIR) was 1.16 (95% confidence interval [CI] 1.15–1.16). There was a statistically significant excess of cancers of lip (SIR = 1.40, CI = 1.28–1.53), liver (SIR = 2.44, CI = 2.35–2.53), pancreas (SIR = 1.75, CI = 1.70–1.79), stomach (SIR = 1.22, CI = 1.18–1.26), colon (SIR = 1.22, CI = 1.19–1.25), gallbladder and bile ducts (SIR = 1.29, CI = 1.21–1.36), non-melanoma skin (SIR = 1.18, CI = 1.15–1.22), kidney (SIR = 1.42, CI = 1.37–1.47), bladder (SIR = 1.17, CI = 1.13–1.21), and thyroid (SIR = 1.22, CI = 1.12–1.31). There was a small statistically significant decrease in prostate cancer incidence (SIR = 0.95, CI = 0.93–0.96). This study showed an association between type 2 diabetes mellitus and the incidence of cancer at numerous sites in the Finnish population.
