Diabetes is a serious and costly disease that is becoming increasingly common in many countries. The role of diabetes as a cancer risk factor remains unclear. To examine the relationship between fasting serum glucose and diabetes and risk of all cancers and specific cancers in men and women in Korea. Ten-year prospective cohort study of 1,298,385 Koreans (829,770 men and 468,615 women) aged 30 to 95 years who received health insurance from the National Health Insurance Corp and had a biennial medical evaluation in 1992-1995 (with follow-up for up to 10 years). Death from cancer and registry-documented incident cancer or hospital admission for cancer. During the 10 years of follow-up, there were 20,566 cancer deaths in men and 5907 cancer deaths in women. Using Cox proportional hazards models and controlling for smoking and alcohol use, the stratum with the highest fasting serum glucose (> or =140 mg/dL [> or =7.8 mmol/L]) had higher death rates from all cancers combined (hazard ratio [HR], 1.29; 95% confidence interval [CI], 1.22-1.37 in men and HR, 1.23; 95% CI, 1.09-1.39 in women) compared with the stratum with the lowest level (<90 mg/dL [<5.0 mmol/L]). By cancer site, the association was strongest for pancreatic cancer, comparing the highest and lowest strata in men (HR, 1.91; 95% CI, 1.52-2.41) and in women (HR, 2.05; 95% CI, 1.43-2.93). Significant associations were also found for cancers of the esophagus, liver, and colon/rectum in men and of the liver and cervix in women, and there were significant trends with glucose level for cancers of the esophagus, colon/rectum, liver, pancreas, and bile duct in men and of the liver and pancreas in women. Of the 26,473 total cancer deaths in men and women, 848 were estimated as attributable to having a fasting serum glucose level of less than 90 mg/dL. For cancer incidence, the general patterns reflected those found for mortality. For persons with a diagnosis of diabetes or a fasting serum glucose level greater than 125 mg/dL (6.9 mmol/L), risks for cancer incidence and mortality were generally elevated compared with those without diabetes. In Korea, elevated fasting serum glucose levels and a diagnosis of diabetes are independent risk factors for several major cancers, and the risk tends to increase with an increased level of fasting serum glucose.