We investigated postoperative mortality in relation to socioeconomic status (SES) in electively operated colorectal cancer patients, and evaluated whether social inequalities were explained by factors related to patient, disease or treatment. Data from the nationwide database of Danish Colorectal Cancer Group were linked to individual socioeconomic information in Statistics Denmark. Patients born before 1921 and those having local surgical or palliative procedures were excluded. A total of 7160 patients, operated on in the period 2001-2004, were included, of whom 342 (4.8%) died within 30 days of surgery. Postoperative mortality was significantly lower in patients with high income (odds ratio (OR)=0.82 (0.70-0.95) for each increase in annual income of EUR 13,500), higher education versus short education (OR)=0.60 (0.41-0.87), and owner-occupied versus rental housing (OR)=0.73 (0.58-0.93). Differences in comorbidity and to a lesser extent lifestyle characteristics accounted for the excess risk of postoperative death among low-SES patients.