In the present study results of serum CA 15-3 immunoassay obtained at diagnosis in 231 breast cancer women (average age: 54.6, range: 27-87 years) were correlated with prognostic factors of the disease; the cut-off level was established at 30.0 U/ ml. As a result, elevated mean values of serum CA 15-3 as well as positivity rates of the test were significantly associated with more advanced stage of breast cancer, presence of distant metastases, involvement of four and more axillary lymph nodes, high BLOOM and RICHARDSON grade [3], low contents of estrogen (ER) and progesterone (PgR) receptors. Although serum CA 15-3 concentrations should be paralleled the increasing tumor size, the difference being significant only for the proportion of positive results. Our findings suggest that pretreatment levels of CA 15-3 antigen represent the breast cancer extent and reflect the cell differentiation and aggressiveness of the tumor. We conclude that pretreatment concentrations of CA 15-3 antigen may be useful as a prognostic factor in breast cancer patients.