Serum CA 15-3 concentrations were determined using sandwich enzyme immunoassay in 430 women: 214 breast cancer patients prior to any therapy, 161 patients with benign breast diseases, and 55 healthy controls; the cut-off limit was established at 30.0 U/ml. In breast cancer patients, CA 15-3 levels positively correlated with negative prognostic factors: higher tumor size (p < 0.001), positive axillary lymph nodes (p < 0.02), high histological grade (p < 0.01), low contents of estrogen (p < 0.05) and progesterone (p < 0.006) receptors. However serum CA 15-3 values raised in parallel with clinical stage of breast cancer, the difference was not significant. The overall diagnostic sensitivity and specificity of the test were 24.3% and 94.9%, respectively. The mean serum CA 15-3 values and the percentage of positive results in breast cancer patients were significantly higher as compared to benign breast diseases group (27.52 +/- 27.01 vs. 16.75 +/- 8.43, p < 0.001; 24.3% vs. 5.6%, p < 0.001, respectively) as well as to healthy controls (27.52 27.01 vs. 13.37 +/- 6.51, p < 0.001; 24.3% vs. 3.6%, p < 0.01, respectively). The sensitivity of the CA 15-3 test is low and thus not suitable for the differential diagnosis of breast lumps. Our data suggest potential prognostic value of pretreatment CA 15-3 assays in breast cancer patients.