In developed countries the growing proportion of elderly colorectal cancer patients with comorbidity will probably complicate clinical management. The aim of this study was to investigate the prevalence of prognostically relevant comorbidity in unselected colorectal cancer patients diagnosed in the Eindhoven Cancer Registry, according to age, gender and subsite and the association with stage of disease, treatment and short-term survival. Comorbid conditions were recorded, according to Charlson's index. The most common concomitant illnesses were cardiovascular diseases, previous cancers and hypertension. The prevalence of comorbidity, especially of cardiovascular disease, previous cancer and diabetes, was highest in the ascending colon. It was slightly higher in patients with Dukes' stage A, probably due to early detection because of regular monitoring for the comorbid condition. Comorbidity was not associated with the resection rate, but was negatively associated with short-term survival. Elder male colorectal cancer patients particularly suffer from substantial comorbidity, influencing the prognosis.