Tenascin is an extracellular matrix glycoprotein expressed during morphogenesis in embryonal life. It reappears in the stroma of benign and malignant tumors. The distribution of tenascin in variants of fibrocystic disease and infiltrating breast carcinoma was assessed in cryostat sections by immunofluorescence using a polyclonal antibody. The tenascin immunoreactivity was compared with various prognostic factors. In fibrocystic disease (n = 10), tenascin appeared as periductal and periacinar bands. In infiltrating carcinomas (n = 32) the tenascin expression was markedly increased. Tenascin immunoreactivity was noted around the ducts (78%), extended into the distal stroma (56%), or was distributed in smaller (reticular) septa around and within tumor-cell nests (34%). Nineteen percent of infiltrating carcinomas did not express tenascin. None of the patterns correlated with prognostic factors such as nodal metastasis, tumor necrosis, invasion of blood vessels, or with flow cytometry results, such as ploidy and S-phase fraction. However, a significantly higher reticular and periepithelial tenascin expression was noted in cases with increased stromal inflammatory reaction. These findings indicate that the appearance of tenascin is neither an indicator of malignancy nor predictive of invasiveness or metastasis but that it is related to local inflammatory response.