Background: Aims of this study were to describe the prevalence of comorbidity in newly diagnosed elderly cancer cases compared with the background population and to describe its influence on overall and cancer mortality. Methods: Population-based study of all 70 year-olds in a Danish province diagnosed with breast, lung, colorectal, prostate, or ovarian cancer from 1 January 1996 to 31 December 2006. Comorbidity was measured according to Charlson's comorbidity index (CCI). Prevalence of comorbidity in newly diagnosed cancer patients was compared with a control group by conditional logistic regression, and influence of comorbidity on mortality was analysed by Cox proportional hazards method. Results: A total of 6325 incident cancer cases were identified. Elderly lung and colorectal cancer patients had significantly more comorbidity than the background population. Severe comorbidity was associated with higher overall mortality in the lung, colorectal, and prostate cancer patients, hazard ratios 1.51 (95% CI 1.24-1.83), 1.41 (95% CI 1.14-1.73), and 2.14 (95% CI 1.65-2.77), respectively. Comorbidity did not affect cancer-specific mortality in general. Conclusion: Colorectal and lung cancer was associated with increased comorbidity burden in the elderly compared with the background population. Comorbidity was associated with increased overall mortality in elderly cancer patients but not consistently with cancer-specific mortality.