Psychiatric disorders were studied in an epidemiological and representative sample of 1964 pupils attending the first to ninth grade in various types of schools in the Canton of Zürich, Switzerland. Using a two-stage procedure, parents were asked to respond to a standardized behaviour problem checklist in the screening phase. In the second stage, structured interviews were performed with 399 parents representing both screen positive and control children. DSM-III-R diagnoses were derived from the interviews. The total prevalence figure for any disorder amounted to 22.5%. Prevalence rates were influenced by the multi-screen procedure. Among various correlates, sex, nationality, age and residence were to some extent significant. Comorbid disorders were present in 12.5% of the subjects.