Objective The vast body of epidemiological literature has suggested the risk of cancer in people with diabetes. As several mechanisms of cancer incidence, such as insulin resistance, hyperinsulinemia, increased insulin-like growth factor have been proposed, there have been growing concerns about the long-term effect of insulin on the risk of several types of cancer. The aim of this study was to investigate the relationship between insulin therapy and cancer incidence in patients with diabetes in Korea. Methods We analyzed the database of 4,537 DM patients who registered in the Korean National diabetes Program (KNDP) from April 1st, 2006 to March 31st, 2011. We included those who had been diagnosed with any type of cancer since one year after the cohort registration. Overall, 3,445 patients were included in the analysis. We classified insulin users according to the types of insulin used. Data were analyzed by Cox's proportional hazard models adjusting for, but not limited to, demographic factors, insulin use, and metformin use. Cumulative cancer incidence was calculated by Kaplan-Meier method. Results All insulin users had longer diabetes duration, worse glucose control, were less obese, and involved to microvascular diabetic complications more. We observed 142 incident cancers (4.1 %, 12.2 per 1000 person years). The most frequent site of cancer in the diabetic cohort was the prostate, followed by colon in men and the thyroid, followed by liver in women. There was no difference in the total number of cancer incidence between any types of insulin users and non-insulin users. Any type of insulin use was not associated with cancer incidence after adjusting for sex, age, metformin use, diabetes duration, glucose control, and body mass index (BMI). Conclusion The association between insulin therapy and cancer incidence was not observed in the Korean diabetes patient cohort. Longer-term large studies are needed.