The primary aim of postoperative surveillance of patients with carcinoma of the colon and rectum is to detect recurrent tumor when cure is still possible. Most recurrences are detected within 30 months after the initial operation. Patients who have had carcinoma of the colon and rectum must be observed not only because of the risk of recurrence or metastatic disease but also because of the increased risk of subsequent primary carcinomas of the colon and rectum as well as of other sites. Careful history-taking and thorough physical examination provide the first indication of tumor recurrence in as many as 48 per cent of instances. Although the liver is the most common site of metastases from carcinoma of the colon, liver chemistry tests are seldom the first to indicate recurrent disease. Fecal occult blood testing, roentgenography with barium enema and colonoscopy are useful surveillance tools, not for detecting recurrences but for detecting second primary carcinomas. Imaging techniques, such as intravenous pyelography, CT and scintigraphy of liver and spleen are generally not cost-effective in surveillance, but MRI and ultrasonography have shown some promise in detection of recurrence without exposing patients to ionizing radiation. The most effective indicator of recurrent disease is a progressive increase in serial levels of CEA. When CEA levels rise and other methods of imaging cannot account for the change, second-look operation is generally appropriate.