Comorbidity is defined as the presence of one or more diseases in addition to an index disease. In elderly people, the number and severity of comorbidity increase with age. We report the comorbidity data of 536 patients treated as in-patients: 231 elderly cancer patients (ECP), 172 younger cancer patients (YCP) and 133 elderly patients admitted for non-cancer reasons (EMP). Comorbidity was assessed with the cumulative illness rating scale geriatric version (CIRS-G). Data on number of affected organ systems (levels 1-4), number of affected organ systems with severe disease (levels 3-4), and sum score of levels per patient are reported. The number of comorbidities increases with age. A 76% of ECP, 51% of YCP, and 79% of EMP have severe comorbidity. Palliative treatment approach is not associated with higher levels of comorbidity in ECP. Vascular disorders were the most common comorbidity. The difficulty to rate haematological comorbidity in cancer patients is reflected. This is the first report on detailed results of assessment of comorbidity measured by CIRS-G in cancer patients. In addition, we provide a comparison to an elderly group of patients admitted for non-cancer reasons.