Worldwide, over 250 million people live with diabetes mellitus, a disease which has been described as an epidemic. While much is known about the cardiovascular risks associated with this condition, its association with cancer has been less appreciated at the population level. This study assessed the relationship between colo-rectal, breast and prostate cancer mortality, and diabetes prevalence measured years earlier at the county level in the 100 counties of North Carolina. Methods: County level data were obtained from the 2000 US Census, the Centers for Disease Control and Prevention, and the North Carolina State Center for Health Statistics. The 2004 diabetes prevalence data by county and county level mortality from 2005 to 2009 for colorectal, breast and prostate cancer mortality were analyzed descriptively. Then multivariate linear regression was carried out to evaluate the contribution of pre-existing diabetes prevalence to cancer mortality, controlling county level covariates. Results: Average North Carolina county level prevalence of diabetes mellitus in 2004 (9%) was higher than the average prevalence of diabetes nationally in 2004 (7%). Mortality rates for breast, colorectal and prostate cancers at the county level were also higher than those nationally. In multivariate analysis, county level 2005-2009 total cancer mortality as well as mortality from colon and prostate cancers (but not breast cancer) correlated with county level 2004 diabetes prevalence rates. Diabetes mellitus prevalence in 2004 explained 31%, 34% and 42% of the variance of mortality from prostate, colorectal and total cancers. Conclusions: These findings support the relationship between diabetes mellitus and cancer at the population level. Direct and indirect costs of cancer care in North Carolina in 2004 were $5.57 billion. Because diabetes explained 42% of the variance of total county level cancer mortality, prevention and control of diabetes could save the state over $2 billion.