A considerable number of tumour markers and other laboratory tests are used for follow up and evaluation of breast cancer patients. No marker presently available is sensitive enough for early diagnosis and screening, but markers can be used to evaluate response to therapy and for early detection of a relapse. Of the markers presently available, CA 15-3 and CEA are mostly used. CA 15-3 appears to be somewhat more sensitive than CEA. When a cut-off level giving 97.5% specificity is used, the sensitivity of these assays is only about 5-25% in local disease and 50-70% in advanced disease. The combined use of these markers increase the sensitivity by 5-10%. The CA 15-3 assay measures a family of molecules called breast mucins, and several newer assays including MCA, MSA, CA 549, CA M26, CA M29 and BCM appear to measure members of the same family. The results obtained by these assays and with CA 15-3 correlate fairly well. So far none of the new assays has proven superior to CA 15-3. Of patients with advanced cancer, 20-30% are negative for the above mentioned markers. For these patients it may be worthwhile to try other markers not related with the above mentioned.