Background. Based on prevalence studies and the few incidence studies of pervasive developmental disorders (PDDs) the prevalence and incidence of these disorders have been claimed to be increasing.

Method. The annual and age-specific prevalence and incidence rates of childhood autism, atypical autism, Asperger's disorder, and pervasive developmental disorder not otherwise specified (PDD-NOS) in Denmark during the period 1971–2000 in children younger than 10 years were estimated using data from the Danish Psychiatric Central Register.

Results. A total of 2·4 million children younger than 10 years were followed and 2061 cases with the PDDs studied were identified. Generally, the prevalence and incidence rates of the PDDs studied were stable until the early 1990s after which an increase in the occurrence of all disorders was seen, until 2000. The annual incidence rate per 10000 children younger than 10 years was 2·0 for childhood autism, 0·7 for atypical autism, 1·4 for Asperger's disorder, and 3·0 for PDD-NOS in 2000. We calculated a ‘corrected’ prevalence of childhood autism at 11·8, atypical autism at 3·3, Asperger's disorder at 4·7, and PDD-NOS at 14·6 per 10000 children younger than 10 years on 1 January 1 2001.

Conclusions. We found that the estimated prevalences of the PDDs studied were probably underestimated. Furthermore, the increasing prevalence and incidence rates during the 1990s may well be explained by changes in the registration procedures and more awareness of the disorders, although a true increase in the incidence cannot be ruled out.