The goal of this research is to ascertain whether a policy of CEA-prompted second-look surgery following "curative" resection of colorectal cancer will produce a decrease in morbidity and mortality due to tumor recurrence. A multicenter, randomized, controlled clinical trial involving 1,500 patients recruited over 3 years and followed for 5 years is proposed. In the 750 patients in the "CEA-Aggressive" arm, regular CEA assays will be used to try to detect recurrent tumor while the patient is still asymptomatic; after a significant CEA rise has been recorded, and screening has excluded widespread disease or a nonmalignant cause for the CEA rise, second-look surgery to locate and remove any treatable recurrence will be mandatory. In the "Conventional Arm," clinical criteria alone will be used by surgeons to detect recurrence and initiate treatment. The trial will be coordinated from the CRC Trials Centre at King's College Hospital, London.