The purpose of this study was to document the prevalence of diabetes among newly diagnosed cancer patients and to evaluate the influence of diabetes on stage at diagnosis, treatment and overall survival. We performed a population-based analyses of all 58,498 cancer patients newly diagnosed between 1995 and 2002 in the registration area of the Eindhoven Cancer Registry. Stage of cancer, cancer treatment and comorbidities were actively collected by hospital medical records review. Follow-up of all patients was completed until January 1, 2005. Nine percent of all cancer patients had diabetes at the time of cancer diagnosis. The prevalence of diabetes was highest among patients with cancer of the pancreas (19%), uterus (14%) and among young men with kidney cancer (8%). Colon, breast and ovarian cancer patients with diabetes were more often diagnosed with a higher tumour stage (p < 0.05). Patients with diabetes and cancer of the oesophagus, colon, breast and ovary were treated less aggressively compared to those without diabetes (p < 0.05). During the follow-up period 3,902 of 5,555 cancer patients with diabetes died and 29,909 of 52,943 cancer patients without diabetes died. For all cancers combined, in a multivariate cox-regression model, adjusting for age, gender, stage, treatment and cardiovascular disease, patients with diabetes experienced a significant increase in overall mortality (HR = 1.44, 95% CI 1.40–1.49), ranging however from 0 to 40% for different types of cancer, compared to those without diabetes. In conclusion, diabetic cancer patients frequently were treated less aggressively and had a worse prognosis compared to those without diabetes. © 2007 Wiley-Liss, Inc.