Background: Childhood cancer survivors have an increased risk of cardiovascular disease (CVD) and diabetes mellitus. Because diabetes is a potentially modifiable risk factor for CVD in the general population, it is important to understand how diabetes affects the risk of CVD among childhood cancer survivors. Methods: This study examined the risk of CVD among survivors with diabetes and 142,742 population comparison subjects. From the national cancer registries of the 5 Nordic countries, 29,324 one-year survivors of cancer diagnosed before the age of 20 years between 1968 and 2008 were identified. Study subjects were linked to the national hospital registers. The cumulative incidence of CVD was determined with competing risk methods. A Cox proportional hazards model was used to estimate the effects of diabetes and cancer on the hazard of CVD. The interaction between diabetes and cancer was analyzed. Results: Diabetes was diagnosed in 324 of the 29,324 one-year survivors, and CVD was diagnosed in 2108. The hazard of diabetes was 1.7 times higher among survivors than comparison subjects (hazard ratio [HR], 1.7; 95% confidence interval [CI], 1.5-1.9), whereas the HR of CVD was 3.6 (95% CI, 3.3-3.8) 1 to 15 years after the cancer diagnosis and 1.9 (95% CI, 1.8-2.0) after more than 15 years. Individuals with diabetes had a 2.4 times higher hazard of CVD (95% CI, 2.1-2.8) among both survivors and comparison subjects in comparison with individuals without diabetes. Conclusions: Childhood cancer survivors with diabetes have a markedly increased risk of CVD in comparison with survivors without diabetes. However, diabetes does not increase the risk of CVD more in survivors than the general population.