Tenascin-C is an extracellular matrix glycoprotein produced in response to epithelial-mesenchymal interactions during organogenesis and tissue remodelling. It has therefore been proposed as a stromal marker for epithelial malignancy. To test this hypothesis, 30 human lung cancers, presenting a variety of clinicopathological features, and six specimens of normal tissue were examined by Western and Northern blotting of tenascin-C protein and mRNA. The results obtained were: (1) elevated tenascin-C expression was detected in all 30 cases by Western blotting, with mRNA increase in 22 of them; (2) mRNA for a large isoform of tenascin-C, including an alternatively spliced sequence, was expressed in lung cancer tissues but not in normal lungs; and (3) metastasis to lymph nodes was frequently found in cases whose tenascin-C was degraded into small fragments. These results suggest that tenascin-C degradation can be used as a marker for metastatic potential of a tumour.