The present study was aimed at investigationg the usefulness of clinical stage, degree of histopatphological malignancy, p53 protein and PCNA labelded indices in deciding the prognosis, proliferation and infiltration of oral squamous cell carcinoma (SCC). Twenty-seven cases of SCC (seven were recurrent cases) were analyzed and 10 cases of non-specific inflammatory lesion served as control. Malignant scores were obtained with the Anneroth's classification. Proliferative activity of SCC in oral cavity was examined by means of proliferaling cell nuclear antigen (PCNA) and p53 protein immunostaining using DO7, PC10 (Novostra Co., U.K.) as primary antibody and biotinylated linked antibody (LSAB kit, Dako Co., U.S.A.) as secondary antibody. The labeled indices of PCNA and p53 were calculated by computer image analysis system in 400 multiplication microscopic fields. Results obtained were as follows : 1. The mean age of SCC group was 61.2 years. Seven of 20 cases had recurrence in 4 months after the operation, in which no significant correlation were noted between pre-and postoperative clinical stage, malignant scores, p53 protein and PCNA indices. 2. Positive immunostaining was noted in 65% (13 of 20 cases) on p53 protein and in 80% (16 of 20 cases) on PCNA. 3. Mean p53 protein and PCNA labeled indices were 24.71+/-27.86% and 41.49+/-2.51% in SCC group. They were high in SCC group than those in control group, although there was no signficant correlation between the two indices. 4. Mean malignant score was 15.22 0.64 in SCC group. There was no correlation between malignant score and clinical stage, and between malignant score and p53 protein or PCNA indices. 5. There was no signficant correlation between SCC and recurrent SCC group in malignant scores, p53 protein or PCNA indices. These results suggest that p53 and PCNA labeled indices have only limited usefulness in deciding the prognosis, proliferation and infiltration of oral squamous cell carcinoma.