The prevalence of pervasive developmental disorders (PDD) is not well established and needs monitoring. We investigated the prevalence of PDDs in the 1999 nationwide British survey of child and adolescent mental health. A randomized stratified sample of children (n = 12,529) aged 5-15 was generated from Child Benefit Records. Trained interviewers interviewed parents and youths aged 11 or older with a standardized diagnostic interview (Development and Well-Being Assessment) and questionnaire data (Strengths and Difficulties Questionnaire) were obtained from teachers and parents who also completed self-report measures of psychological distress. A team of experienced clinicians using all data sources achieved final diagnostic determination. A total of 10,438 (83%) interviews were conducted. There were two girls with Rett syndrome (weighted prevalence: 3.8/10,000 girls) and 27 children with other PDDs (weighted prevalence: 26.1/10,000). Compared to children with a psychiatric disorder other than PDD, social but not behavioural problems were more frequent in the PDD group. Parents of children with PDDs had higher rates of psychological distress than those from the two comparison groups. Consistent with other recent surveys, PDD rates are higher than those reported 30 years ago. The burden associated with PDDs is very high.