Background: A noticeable proportion of colorectal cancer (CRC) patients are diagnosed with synchronous CRC. Large population-based studies on the incidence, risk factors and prognosis of synchronous CRC are, however, scarce, and are needed for better determination of risks of synchronous CRC in patients diagnosed with colonic neoplasia. Methods: All newly diagnosed CRC between 1995 and 2006 were obtained from the Rotterdam Cancer Registry in The Netherlands, and studied for synchronous CRC. Results: Of the 13,683 patients diagnosed with CRC, 534 patients (3.9%) were diagnosed with synchronous CRC. The risk of having synchronous CRC was significantly higher in men (OR 1.54, 95% CI 1.29-1.84) and in patients aged >70 years (OR 1.83,95% CI 1.39-2.40). Synchronous CRC patients had a significantly higher risk of distant metastases (OR 1.69, 95% CI 1.27-2.26). In 34% (184/534) the two tumours were located in different surgical segments. Five-year relative survival of synchronous CRC was similar to patients with solitary CRC after multivariate adjustment for the presence of distant metastases. Conclusion: One out of 25 patients diagnosed with CRC presents with synchronous CRC. In the multivariate analysis, survival of patients with synchronous CRC was similar to patients with solitary CRC, when corrected for the presence of distant metastases at first presentation. One third of the synchronous CRC were located in different surgical segments, which stresses the importance of performing total colon examination preferably prior to surgery. (C) 2011 Elsevier Ltd. All rights reserved.