The purpose of this study was to determine the prevalence of serious concomitant conditions at diagnosis among unselected patients with cancer, increasingly older in industrialized countries. About 34,000 newly diagnosed cancer patients were recorded in the Eindhoven Cancer Registry between 1993 and 1996; subsequently data on serious co-morbidity, classified according to the Charlson scheme (J Chron Dis 1987; 40: 373-383), were collected from the clinical records by registry personnel. Co-morbid conditions were present in 12% of adult patients below 45 years of age, 28% of those 45-59 years, 53% of those 60-74 years, and 63% of patients over 75 years of age, the prevalence being highest for patients with lung (58%), kidney (54%), stomach (53%), bladder (53%), and prostate cancer (51%). Males exhibited a 10% higher prevalence than females with similar tumors. Among patients over 60 years the most frequent conditions were heart and vascular diseases (ranging across the various tumors from 10% to 30%), hypertension (11-25%), another cancer (10-20%), COPD (chronic obstructive pulmonary disease) (3-25%), and diabetes mellitus (5-25%). Inclusion of frequent co-morbid conditions in prognostic research as well as the development of specific guidelines for patient care seems warranted.