Prior studies of cancer risk among diabetic men have reported inconsistent findings. The aim of this study was to assess the risk of cancer among a large cohort (n = 4,501,578) of black and white U.S. veterans admitted to Veterans Affairs hospitals. The cancer risk among men with diabetes (n = 594,815) was compared to the risk among men without diabetes (n = 3,906,763). Poisson regression was used to estimate adjusted relative risks (RRs) and 95% confidence intervals (CIs). Overall, men with diabetes had a significantly lower risk of cancer (RR = 0.93, 95%CI = 0.93-0.94). Men with diabetes, however, had increased risks of cancers of the liver (RR = 1.95, 95%CI = 1.82-2.09), pancreas (RR = 1.50, 95%CI = 1.42-1.59), biliary tract (RR = 1.41, 95%CI = 1.22-1.62), colon (RR = 1.20, 95%CI = 1.16-1.25), rectum (RR = 1.12, 95%CI = 1.07-1.18), and kidney (RR = 1.09, 95%CI = 1.03-1.16), as well as leukemia (RR = 1.14, 95%CI = 1.08-1.21) and melanoma (RR = 1.13, 95%CI = 1.03-1.24). In contrast, men with diabetes had decreased risks of cancers of the prostate (RR = 0.89, 95%CI = 0.87-0.91), brain (RR = 0.91, 95% CI = 0.82-0.99), buccal cavity (RR = 0.85, 95%CI = 0.82-0.89), lung (RR = 0.79, 95%CI = 0.77-0.80), esophagus (RR = 0.77, 95%CI = 0.72-0.82), and larynx (RR = 0.76, 95%CI = 0.71-0.80). These findings indicate that black and white men with diabetes are at significantly lower risk of total cancer and of two of the most common cancers among U.S. males; lung and prostate cancers. These decreased risks were offset, however, by increased risks of cancer at several sites. Hyperinsulinemia may explain the increased risks of the digestive cancers, while lower testosterone levels, in the case of prostate cancer, and higher BMI, in the case of lung cancer, may explain the decreased risks of those tumors.