To investigate the changes of cytokeratin 19 during oral carcinogenesis.53 specimens including normal oral mucosa, oral epithelial hyperplasia, oral epithelial dysplasia and oral squamous cell carcinoma were investigated by immunohistochemistry, SDS-PAGE and Western blotting.CK19 was detectable in suprabasal cell layers in epithelial dysplasia and in oral cancer, especially in poor-differentiated cancerous cells. With the lesions getting worse, the positive rate, the intensity and the constituent ratio of CK19 raised significantly.The results suggest that the CK19 expression in suprabasal cell layers of oral mucosa can be used as a marker of diagnosis of oral precancerous lesions and CK19 expression is the initial events during oral carcinogenesis.