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Insight into the prevalence of pervasive developmental disorder (PDD) in children and adolescents with mental retardation (MR) is known to be of clinical importance. However, estimating this prevalence is complicated. The literature reports prevalence rates ranging from 3% through 50%. This variation seems to be related to the concepts of PDD under study, the instruments used, and the studied populations. The present study aimed to estimate a reliable prevalence rate of PDD.A total population-based screening with the PDD-MRS and the ABC (n = 825) was followed by further assessment of children and adolescents at high risk for PDD according to these instruments, and for controls, with the ADI-R, ADOS-G and a DSM-IV-TR classification (n = 188).The instruments lead to different prevalence rates that range from 7.8% to 19.8%. The differences in the estimated prevalence rates are related to the concept of PDD and the instruments they represent. The DSM-IV-TR prevalence (16.7%) seems to be the most reliable and well-founded estimate, since this prevalence rate is based on information from multiple informants and multiple time periods.The reported prevalence rates provide policy makers with an up-to-date and more substantiated guideline for the allocation of resources for children and adolescents with MR and PDD. The height of the prevalence should alert professionals that PDD is widespread in the population with MR.