We investigated how oral epithelial dysplasia (OED) relates to the expressions of Pcadherin and cyclin D1 using biopsy specimens of 32 cases of OED, with 20 cases of oral squamous cell carcinoma (OSCC) as controls. In normal oral epithelium and mild OED, the expression of P-cadherin is observed in the cell membrane of the basal cell layer, except on the basement membrane side, and the expression of cyclin D1 is observed in the nuclei of a few basal cells. We observed strong expression of P-cadherin in the cell membrane and cytoplasm of most cells with severe OED and in cancer cells with OSCC. There was a significant correlation between the expression of P-cadherin and the severity of OED (p<0.05). Although the expression of cyclin D1 was observed in nuclei of OED cells and OSCC cells, there was no correlation between the expression of cyclin D1 and the severity of OED (p=0.55).<br> These results suggest that P-cadherin is involved in cell proliferation. (J Osaka Dent Univ 2005; 39: 97-101)