We investigated endotumoral and peritumoral lymphocytic subsets [natural killer cells (NK), B-cells and cytotoxic/suppressor (CD(8)+) T-cells], and expression of MUC1 and MUC6 glycoprotein with regard to various clinicopathological parameters in invasive breast cancer tissues. The study population consisted of 64 female patients with invasive ductal breast cancer of not-otherwise-specified type. Thirty-five women with benign breast lesions served as controls. High-grade carcinomas exhibited higher numbers of endotumoral NK cells and B-cell aggregates than the rest of the tumors examined (P=0.0003 and 0.027, respectively). Cases with more than three positive lymph nodes and with tumors over 2 cm in diameter exhibited higher numbers of endotumoral NK cells (P=0.047 and 0.023, respectively). Increased numbers of peritumoral CD(8)+ T-cells were detected in cases with lymph node metastases (P=0.045). MUC1 was expressed with weaker staining intensity in the control group than in the group with breast cancer (P=0.011). Grade III carcinomas exhibited significantly stronger expression of MUC6 glycoprotein (P=0.001) than the control group. In conclusion, tumors with markers of poor prognosis exhibited increased numbers of lymphocytic infiltrates, and of NK cells in particular, and stronger MUC1 and MUC6 glycoprotein immunoreactivity than did the other tumors.