Approximately two-thirds of all colorectal cancer patients undergo surgery with the aim of curing them. However, despite the surgery, 20-25% of them experience relapse. It is possible to reduce the risk of relapse with chemotherapy, but as chemotherapy is associated with significant side effects, it is only given to patients at high risk of relapse. Currently, the risk is assessed based on an examination of the removed tumor tissue. In a previous research project, blood samples were taken after patients' surgery and examined for the presence of circulating tumor DNA (ctDNA). When cancer cells in solid tumors die, they release DNA, which can be detected in the blood. DNA in the blood has a half-life of less than 2 hours, so if ctDNA is found in a blood sample taken, e.g., 14 days after surgery, the patient most likely still has cancer cells in their body. The results show that if a patient has ctDNA in their blood after surgery, the risk of relapse is high. The presence of ctDNA in the blood has the potential to be a better indicator of the risk of future relapse than the tumor examination used today. Therefore, ctDNA analysis has the potential to become a marker that will be used in the future clinical setting for monitoring colorectal cancer. The overall objective of this study is to confirm that ctDNA found in a blood sample after intended curative treatment for CRC is a marker of residual disease and risk of recurrence and is applicable in clinical practice.