Oral mucosal lesion findings from a national multistage probability oral health survey of United States schoolchildren in kindergarten through grade 12 are reported. In the 1986-87 school year 39,206 children aged 5-17 yr were examined by 14 dentists trained in standardized clinical diagnostic criteria for dental caries, periodontal conditions and oral mucosal lesions. In addition all children were asked whether or not they ever had "cold sores," "fever blisters," or "canker sores", and adolescents (grades 6-12) were questioned about their history of tobacco use. About 4% of the children had one or more oral mucosal lesions present at the time of the examination, while 33 and 37% reported a history of recurrent herpes labialis and recurrent aphthous ulcers, respectively. The most prevalent lesions clinically observed were recurrent aphthous ulcers (1.23%), recurrent herpes labialis (0.78), smokeless tobacco lesions (0.71), and geographic tongue (0.60). Differences in prevalence were analyzed by age, sex, race, metropolitan area, and geographic region. Almost 10% of 12-17-yr-olds reported current use of some type of tobacco product. In adolescents the current use of tobacco products had a marked effect on the prevalence of oral lesions.