ABSTRACT. A triaxial model for autistic spectrum disorder (ASD) is presented, incorporating age, degree of intelligence, and severity of autistic features. As the name implies, ASD can vary in degree of expression from minimal to profound. Furthermore, the symptoms of ASD change in predictable ways with the passage of time. For example, echolalia during early childhood may be replaced by verbal literalism and difficulty with verbal humor during later childhood or adolescence. The prognosis for children with ASD is governed by the joint impact of the degree of expression of ASD and the degree of developmental delay, if any. All combinations of ASD and intellect are possible (ie, severe ASD plus severe mental retardation, severe ASD plus normal general intelligence, and so forth). The relationship among these 3 parameters-severity of ASD, level of general intelligence, and change in symptom expression over time, is represented schematically as a 3-dimensional graph. The utility of this graph as a counseling tool, and as the basis for future research on the prognosis of ASD are discussed. Pediatrics 2000;105(5). URL: http://www. pediatrics.org/cgi/content/full/105/5/e65; autism, autistic spectrum disorder, pervasive developmental disorder, Asperger syndrome, developmental disability.ABBREVIATIONS. ASD, autistic spectrum disorder; MR, mental retardation. P arents of the child with autistic spectrum disorder (ASD) commonly desire information regarding prognosis. In this article, I present a conceptual model, based on 2 decades of clinical experience, that I have found useful when introducing parents to the diagnosis of ASD and making preliminary statements about prognosis. This model does not enable the clinician to specify a given child's outcome. Rather, it conveys an overall perspective of the range of expression of ASD and acts as a roadmap on which the parents may plot their own child's progress over time. Although based primarily on my clinical experience, I believe this model also has the potential to serve as a frame of reference for formulating researchable questions regarding the epidemiology and prognosis of ASD, terminological issues, and the putative benefits of various therapeutic interventions.The qualitative distortions of development in ASD are "atypical," in the sense that there is no age for which they would constitute typical phenomena. For example, emergence of single words at 24 months of age would be indicative of speech delay (abnormally late, but typical behavior for a younger child); in contrast, absence of speech until 24 months of age, followed by the sudden appearance of echolalia and delayed echolalia would be considered atypicalthere is no age for which this constitutes normative behavior. One parent of a child with ASD articulated this concept succinctly, describing her son as "off the beaten path."The model I use when counseling parents rests on 4 generally accepted premises:• First, atypical development occurs along a spectrum from mild to severe, although differences of opinion rema...