Comorbidity has large impact on colorectal cancer (CRC) treatment and outcomes and may increase as the population ages. We aimed to evaluate the prevalence and time trends of comorbid diseases in patients with CRC from 1995 to 2010. The Eindhoven Cancer Registry registers comorbidity in all patients with primary CRC in the South of the Netherlands. We analyzed the prevalence of serious comorbid diseases in four time frames from 1995 to 2010. Thereby, we addressed its association with age, gender and socio-economic status (SES). The prevalence of comorbidity was registered in 27,339 patients with primary CRC. During the study period, the prevalence of comorbidity increased from 47% to 62%, multimorbidity increased from 20% to 37%. Hypertension and cardiovascular diseases were most prevalent and increased largely over time (respectively 16-29% and 12-24%). Pulmonary diseases increased in women, but remained stable in men. Average age at diagnosis increased from 68.3 to 69.5 years (p 5 0.004). A low SES and male gender were associated with a higher risk of comorbidity (not changing over time). This study indicates that comorbidity among patients with CRC is common, especially in males and patients with a low SES. The prevalence of comorbidity increased from 1995 to 2010, in particular in presumably nutritional diseases. Ageing, increased life expectancy and life style changes may contribute to more comorbid diseases. Also, improved awareness among health care providers on the importance of comorbidity may have resulted in better registration. The increasing burden of comorbidity in patients with CRC emphasizes the need for more focus on individualized medicine.Comorbidity composes a great challenge when treating patients with colorectal cancer (CRC). [1][2][3][4] In the Netherlands, CRC represents the second most frequent cancer in terms of incidence with more than 12,000 newly diagnosed patients annually and a lifetime risk of more than 5%. 5 As more than half of patients with CRC is aged older than 70 years, the diagnosis of CRC is often made amidst the presence of other chronic medical conditions. Treatment of patients with severe comorbidity is challenging because of polypharmacy and decreased compensating mechanisms, especially in older patients who also have normal age-related physiological changes. 6 The presence of a single or combination of chronic illnesses can affect both treatment effectiveness and tolerance, and is associated with worse short-and long-term outcomes after CRC surgery. [1][2][3][4][7][8][9][10][11] The prevalence of comorbidity is influenced by personal and environmental factors. Age, gender and socio-economic status (SES) have been described as interacting with the burden of specific comorbid ailments and influencing outcomes after CRC treatment. [12][13][14] Further, ageing, improved life expectancy and lifestyle habits in western countries will lead to a higher prevalence of (multiple) concomitant diseases among patients with CRC. 15,16 Objectifying increases in the burden of comorbi...