Involucrin is a keratinocyte envelope protein precursor which is synthesized at an early stage of differentiation in normal squamous epithelium. Recent studies suggest that this protein may be a marker for neoplastic epithelium. To address this issue, we analyzed involucrin expression in 105 biopsies containing 119 areas of normal, condylomatous, and neoplastic epithelium. Overall, 88, 75, and 55% of condylomata, well-differentiated CIN, and poorly differentiated CIN (carcinoma in situ) contained positive staining for involucrin. Excluding lesions with severe inflammation, 100, 88, and 55% of these lesions, respectively, were positive. Staining patterns in neoplastic lesions differed from those in the normal epithelium and condylomata; the staining in CIN tended to be focal, and intensity of staining varied widely from cell to cell in all layers of the epithelium. In high grade CIN, staining correlated with increases in cell size and cytoplasmic differentiation. These studies suggest that involucrin will not differentiate between lesions of low versus high risk for progressing to invasive carcinoma. However, the patterns of involucrin expression confirm the marked differences in patterns of cellular differentiation between classical condylomata and CIN.