The development of head and neck cancer has been proposed to be a multistep process, with accumulation of genetic and phenotypic alterations resulting from carcinogen exposure. Proliferating cell nuclear antigen (PCNA), also called cyclin, is a 36-KD auxiliary protein of DNA polymerase-delta, that has been found to be a useful marker in immunocytochemical studies of cell proliferation because its expression correlates with the proliferative state of the cell. PCNA expression was analyzed in 10 samples of normal mucosa, 23 benign oral lesions (18 hyperplasia and 5 oral lichen plani), 10 oral lesions with epithelial dysplasia, and 10 dysplastic epithelia adjacent to tumors. Immunocytochemical stained sections were scored for the presence or absence of suprabasal PCNA positivity regardless of location. The results indicate that the PCNA expression in the suprabasal layers increased with the degree of epithelia dysplasia and in the samples of histological dysplastic epithelium adjacent to the tumors, while the percentage of suprabasal PCNA expression was insignificant in the samples of normal oral mucosa and benign oral lesions. The authors conclude that suprabasal PCNA expression could be a marker of dysplasia in oral mucosa, indicating a special proliferative cellular state in those lesions.