The potential usefulness of MCA, CA 15-3 and CEA in monitoring of breast cancer patients was evaluated in 135 female patients with histologically confirmed breast cancer. The patients were classified into two groups as follows: group of patients with no evidence of disease, NED; and group of patients with progressive disease, PD. In total, 2106 measurements of CEA, CA 15-3, and MCA were performed using an enzyme immunoassay. Serum levels of all three markers in the NED group differed significantly from those of patients with PD. The observed differences in the sensitivity and specificity of CEA, CA 15-3, and MCA tests were not significant. The serum concentrations of a particular marker correlated well with the concentrations of the other two markers, except when CEA was correlated with MCA or CA 15-3 in NED group patients. The elevation of tumor markers preceded by some 7 months the clinical evidence of dissemination, and marker levels reflected at a high percentage the response to therapy in PD patients. Therefore, this clinical study confirmed that MCA, CA 15-3 and also CEA are suited to discriminate between disease and disease-free periods, and also validated the usefulness of markers for treatment response monitoring.