The aim of this work was to review the usefulness of tumour markers in the follow-up of colo-rectal cancers after operation. The carcinoembryonic antigen (CAE) is the most sensitive marker for early detection of relapse. It can be combined with CA 19-9 assay, increasing the sensitivity and specificity of the tests. These assays are useful because one half of the relapsing tumours can be resected leading to a survival rate of 40%. CAE and CA 19-9 must be assayed within 2 months to 2 years after surgery to be effective screening tests. Presently, the widespread use of labelled monoclonal antibodies can, in certain tests, localize recurrent tumours before a second look operations.