An association between diabetes mellitus (DM) and cancer has long been speculated. Inoue et al prospectively examined this association in 97 771 Japanese men and women aged 40 to 69 years, who were followed up from 1990 through 2003. A total of 6462 cases of newly diagnosed cancer were identified. In men, a 27% increase in the risk of total cancer incidence was observed in those with a history of DM. The risk was especially high for cancer of the liver, pancreas, and kidney. In women, a 21% increase in risk was observed for the incidence of total cancer, and the risk was especially high for cancer of the stomach, liver, and ovary. These findings indicate that patients with DM drawn from the general Japanese population may be at increased risk of total cancer and of cancer in specific sites.