['Type XIII collagen (ColXIII) is a transmembrane protein thought to be involved in cell-cell and cell-matrix interactions. We report here on its presence in the normal human cornea and compare the results for keratoconus and scarred corneas.', 'Immunohistochemistry and in situ hybridization were applied to human corneal samples obtained by penetrating keratoplasty.', 'In the normal human cornea, ColXIII was immunolocalized to the corneal epithelial cells, and to a lesser degree to the stromal keratocytes. The keratoconus cases showed otherwise similar results, but in areas containing Bowman membrane disruptions showed thinned epithelial cells reduced immunostaining for ColXIII, whereas occasionally pronounced immunoreactivity was seen in the stromal keratocytes. The corneal scar samples contained highly increased ColXIII immunostaining by stromal cells in the fibrotic foci, whereas the peripheral areas showed less intense immunostaining. In situ hybridization confirmed that the corneal epithelium and keratocytes actively synthesize the transcript. Immunostaining with alphaSMA revealed that a substantial proportion of the ColXIII mRNA-expressing cells in the stromal scar tissues was myofibroblasts and that these areas lack CD34 immunoreactivity.'] Is altered expression of type XIII collagen in keratoconus and scarred human cornea : Increased expression in scarred cornea associated with myofibroblast transformation?