Pervasive developmental disorders (PDD) refer to the group of heterogeneous conditions that make up a continuum or spectrum of autistic disorders and share a core triad of impairments consisting of qualitative disturbances in social interaction and verbal and nonverbal communication and imagination. It has long been believed that the prevalence of autism was 2-4 per 10,000 children. However, studies using broader definitions of autistic disorder have suggested that the prevalence of autism spectrum disorder may be as high as 90 per 10,000 and that a greater proportion of individuals with PDD have intellectual levels above 70. Clinicians and researchers have commented on the apparent increase in prevalence of the disorder and have offered a number of explanations, including better recognition and diagnosis of the autism spectrum of disorders and a real increase in the disturbance. It is being increasingly recognized that individuals with PDD are at risk for a wide array of psychiatric disturbances, including affective disorder, anxiety disorders, schizophrenia-like psychosis, aggression, antisocial behavior, and Tourette's disorder (TD). Evidence indicates that PDD is significantly related to the comorbid psychiatric disorders and TD. Because PDD is not rare, individuals with autism spectrum disorder may represent significant subgroups of severely emotionally disturbed patients referred for psychiatric treatment. Because of lack of awareness that the clinical manifestations of PDD are heterogeneous and often mild and that comorbid psychiatric disturbances may obscure the symptoms of the developmental disorder, the diagnosis of PDD may be missed. Implications for practicing clinicians are discussed.