During the 27 years between 1962 and 1988, 984 patients visited the National Cancer Center Hospital for previously untreated squamous cell carcinoma of the oral cavity and lip. The records of all these 984 patients were reviewed to determine the incidence of additional primary carcinoma. 1) One hundred and thirty-five additional carcinomas developed in 111 patients (11.2%) during 5,689.2 person-years of observation. The incidence of additional primary carcinoma was 23.7 per 1,000 person-years. 2) The cumulative rate of additional primary carcinoma during the first five years of observation showed a tendency to increase in the most recently treated patients (from 1980 to 1988). 3) The observed-to-expected ratio (O/E ratio) for all sites was 2.77 and this is significantly high (P less than 0.01). The calculation of the O/E ratio for each site revealed significantly high risks in the oral cavity and pharynx, esophagus and skin. The O/E ratio for the oral cavity and pharynx was extremely high (79.45). 4) The O/E ratio for all sites in each year of follow-up was the highest in the first year, stayed nearly constant from the second to 14th years, and decreased gradually afterwards. Significantly high risk was observed until the 13th year of follow-up. Patients with oral squamous cell carcinoma must be under frequent and regular examination for almost 15 years.