The aim of this study was to ascertain accurately the prevalence of Tourette syndrome (TS) in a mainstream school population. All year 9 pupils (aged 13 to 14 years) in a mainstream secondary school were investigated using a two-stage procedure. Standardized questionnaires were completed by parents, teachers, and pupils. Class observations were also carried out to identify tics. Those pupils identified as having tics underwent a semistructured interview to determine whether they had TS according to DSM-III-R criteria. Data were available from at least one source (parent, teacher, or self-reports) on 166 of the 167 pupils in the year. Five subjects were identified as having TS according to DSM-III-R criteria, yielding a prevalence estimate of 299 per 10,000 pupils in this age group. The results of this study suggest that TS in the community as a whole is more common and milder than those prevalence estimates and descriptions of the disorder based on TS encountered in secondary or tertiary health-care service settings.