Is the incidence of autistic disorder and other pervasive developmental disorders (PDDs) increasing? Recent epidemiological surveys of autistic disorder and other PDDs have heightened awareness of and concern about the prevalence of these disorders; however, differences in survey methodology, particularly changes in case definition and case identification over time, have made comparisons between surveys difficult to perform and interpret. Recent surveys suggest that the rate of all PDDs is about 60 per 10,000. The prevalence of autism today is estimated at 13 per 10,000, Asperger's disorder is approximately 3 per 10,000, and childhood disintegrative disorder is very rare at about 0.2 per 10,000. The assessment process, sample size, publication year, and geographic location of studies all have an effect on prevalence estimates. In addition, data from many of these surveys indicate correlates of autistic disorder and other PDDs with IQ, gender, and other medical disorders.