To study the relationship of abnormal expression of mucin 1 with the invasiveness of breast carcinoma cells.Immunohistochemistry was used to detect the protein expression of mucin 1 in 5 specimens of juxta-cancerous normal tissues, 20 specimens of benign breast tumors, 35 specimens of early breast carcinoma, 22 specimens of infiltrating cancerous tissues, and 20 specimens of lymph node foci with metastatic breast carcinoma. Human breast cancer cells of the line MCF-7 were cultured and transfected with antisense oligodeoxynucleotide (ASODN) of mucin 1. The mucin 1 mRNA expression in the cells was detected by RT-PCR and the protein expression of mucin 1 in the cells was detected by flow cytometry. The cell invasiveness was detected by Matrigel invasion assays.Top membrane positive expression of mucin 1 was observed in the normal breast tissues and breast benign tumors and whole membrane positive expression of mucin 1 was observed in the 30 of the 35 specimens of early breast carcinoma, 18 of the 22 specimens of breast infiltrating carcinoma, and 17 of the 20 specimens of lymph node metastatic tissues. The mRNA and protein expressions of mucin 1 in the breast carcinoma cells treated with ASODN of mucin 1 were significantly decreased (both P < 0.05). The number of invasive cells decreased significantly in the cell treated with ASODN of mucin 1 in comparison with those treated with sense nucleotide (P < 0.05).The abnormal distribution of mucin 1 contributes to the invasiveness of carcinoma cells and may not make difference in the lymphogenous metastasis of the carcinoma. The invasiveness of breast carcinoma cells can be inhibited by the ASODN complementary to the start site of mucin1 mRNA.