Oral cancer incidence rates rose dramatically during the twentieth century in the United States and Europe, especially among individuals under the age of 60 years. Although influenced by age, sex, and country of origin, incidence trends were most strongly affected by elevated risk among individuals born after approximately 1915. This cohort effect was indicative of strong behavioral influences on oral cancer risk. In this article, associations between oral cancer risk and established behavioral risk factors including alcohol and tobacco use are reviewed. Additionally, possible associations between oral cancer risk and oral hygiene, diet, nutritional status, and sexual behavior as well as the influence of genetic factors on oral cancer risk are considered. Special emphasis is placed on evaluating possible risk differences in individuals above and below the age of 45 and in users and nonusers of alcohol and tobacco. Oral squamous cell carcinomas arise from several anatomic sites within the oral cavity and oropharynx, but most commonly from the oral, mobile tongue.1 In the 1980s, 2 cancer centers in the United States (U.S.) reported an increase in the proportion of incident oral tongue cancers diagnosed in men younger 40 years.2,3 Although absolute numbers had increased, these reports provided no real evidence of rising incidence. Shortly thereafter, however, a significant 2-fold increase in oral tongue cancer mortality rates between 1950 and 1982 was reported in the U.S. for men younger than 30 years. 4 This finding was corroborated by a study in Scotland. 5 Further analysis of the Scottish data revealed that the increase in incidence was not restricted to those younger than 40 but extended to all age groups less than 65 years of age born after 1920-a strong ''cohort effect.'' The cohort effect was indicated by a strong effect of year of birth on oral cancer incidence rates. A flurry of research on global changes in oral cancer incidence was stimulated by these reports.