Tenascin-C is an extracellular matrix glycoprotein that is markedly upregulated in the dermis of psoriatic skin. In this study, we have addressed the question whether the presence of tenascin-C in the lesion or in serum is a marker for disease activity. Immunohistochemical staining of tenascin-C before and after treatment with different topical and systemic medication showed that tenascin-C remained abundant after clinical remission of lesions, indicating that downregulation of tenascin-C to normal values is a slow process. By using a sensitive enzyme-linked immunosorbent assay to measure levels of serum tenascin-C in psoriatic patients and unaffected individuals, we found that tenascin-C levels in most patients were within the normal range. Moreover, tenascin-C values did not correlate with disease activity. We conclude that tenascin-C is not useful as a marker for disease activity in psoriasis.