SummaryThe expression of tenascin, a recently discovered extracellular matrix glycoprotein, was studied by immunohistochemistry in normal human skin and in a number of skin diseases with epidermal hyperproliferation such as psoriasis, basal cell carcinoma. Bowen's disease and solar keratosis. Tenascin expression in the upper dermis of normal skin was found to vary from almost absent to patchy along the basal membrane. Staining was continuous and intense around blood vessels, hair follicles and eccrine sweat ducts. In basal cell carcinoma a marked expression of tenascin was found in the tumour stroma, especially adjacent to the basal membrane surrounding the tumour cell nests. In Bowen's disease and solar keratosis, tenascin expression was found in the dermis next to the keratinocytes. In psoriasis the dermal papillae of clinically involved skin were intensely stained and a continuous band of tenascin was present in the upper dermis along the basal membrane. The distribution of tenascin differed from other known extracellular matrix components.