Laryngeal keratosis is one of the most well-known premalignant lesions, but its histopathologic classification have not been standardized. Sometimes the keratotic lesions are associated with cellular atypia which has importance in pathology because severe atypia occasionally transforms to invasive carcinoma. Proliferating cell nuclear antigen(PCNA) indicates the cellular kinetic activity during the late Gl and S phase;therefore, PCNA might be related to the degree of cellular atypism. The purpose of this study is to evaluate whether PCNA is a indicator to malignant potential of keratosis or not. The authors investigated the PCNA expression rates by immunohistochemical staining for monoclonal antibody to PCNA in 21 laryngeal keratosis from 1984 to 1992. The results were as follows : 1) The average PCNA expression rate was 6.01±5.67% in laryngeal keratosis, but 0.26±4.29% in normal laryngeal mucosa. 2) The PCNA expression rate was 2.98±3.18% in epithelial hyperplasia, but 9.79±5.82% in epithelial dysplasia. 3) The PCNA expression rate was 15.11±2.14% in mild atypia, 3.51±2.26% in moderate atypia, and 4.22±3.11% in severe atypia according to histopathologic differentiation in epithelial dysplasia. 4) Recurrent cases or the cases of malignant change showed 9.71±4.50% PCNA expression rate as compared with 4.40±3.02% in the cases without recurrences. This study suggests that PCNA expression rate can be useful in detecting laryngeal hyperkeratosis with atypia and as a indicator to predict the prognosis in laryngeal premalignant lesions.