The serum levels of SCC antigen were investigated both in 157 patients with oral squamous cell carcinomas and in 28 patients with oral benign diseases as control. The cut off level could be determined to be 1.71 ng/ml with the mean level of control group plus twice standard deviation. There was a significant correlation between the serum levels and clinical findings of oral carcinomas: the antigen levels closely correlated with the changes of tumor volume and clinical stages in squamous cell carcinoma patients. However, there was no relationship between tumor differentiation and SCC antigen levels. It appeared that higher serum levels of SCC antigen were found in patients with carcinomas of the maxilla and floor of the mouth than in that of other regions. As the serum levels of SCC antigen have increased in the follow-up patients, even if lower than the cut off level, they often suggest the relapses of malignant tumor.