Sections of formalin-fixed, paraffin-embedded tissues from 210 human breast cancers were immunohistochemically examined using the mAb against human tenascin (TN) RCB1. Immunoreactive TN was detected in the breast cancer stroma in 77 (36.7%) cases, whereas the remaining 133 (63.3%) were negative. Of the 77, 12 (5.7%) cases also showed positive staining in the carcinoma cell cytoplasm. The positive cells were often observed in the margin of the cancer nests at the site adjacent to the stroma. According to the staining pattern of TN, the breast cancer cases were classified into the three groups of cancer cell TN(+)/stromal TN(+), cancer cell(-)/stromal TN(+), and cancer cell(-)/stromal TN(-). Analysis of the relationship of these TN patterns with various clinicopathological characteristics of the tumors and the patient outcome revealed that, in comparison to the cancer cell(-)/stromal TN(-) group, the cancer cell TN(+)/stromal TN(+) group exhibited increased frequency of lymph node metastasis and exceptionally poor outcome, and the cancer cell(-)/stromal TN(+) group also showed more frequent metastasis and poorer outcome. Most of the cancer cell TN(+)/stromal TN(+) cases were c-erbB-2 positive and estrogen receptor negative. Furthermore, in situ hybridization of freshly obtained breast cancer tissues demonstrated that both cancer cells and stromal cells express TN mRNA. These results indicate that the TN in breast cancer is produced by cancer epithelial cells as well as by stromal mesenchymal cells, and that cancer cell TN might be involved in cancer spreading, resulting in unfavorable patient prognosis.