There were 6181 cases of invasive intraoral squamous cell carcinoma accessioned by the Connecticut State Tumor Registry from 1935 to 1985. Cases were analyzed for age, sex, lesion site, and histologic differentiation. Crude, age-specific, and age-adjusted incidence rates plus birth cohort analyses were also calculated. It was found that incidence rates for both men and women increased over the 51-year period of study. For men, age-adjusted incidence rates (1970 United States standard) increased from 4.9/100,000 in 1935 to 1939 to 8.5/100,000 in 1980 to 1985; for women, rates increased from 0.5/100,000 to 3.3/100,000 for the same period. The male-to-female ratio for intraoral squamous cell carcinoma declined dramatically from 9.8 to 2.6 during the 51-year study period primarily because of the steep rate of increased incidence in women relative to that seen in men. The peak age of intraoral squamous cell carcinoma was the seventh decade. Age-specific analysis showed that the older the age group, the higher the incidence for both sexes. During recent years, there was evidence of slightly increased incidence in men younger than 40. The tongue was the most common site for intraoral squamous cell carcinoma, followed closely by the floor of the mouth. Moderately differentiated tumors were most common (54.3% of the total), followed by both well-differentiated cases (29.1%) and those that were poorly differentiated (16.6%).