This study evaluated the impact of intensive behavioral treatment on the development of young autistic children. The treatment reported in this study was home based and was implemented by parents of autistic children with the assistance of community-based clinicians. Although treatment was unable to be observed directly, parents reported that therapy was based on methods developed by Lovaas et al. (1981). Treatment differed from that described in previous reports of intensive behavior therapy for this population in that it was implemented outside an academic setting and for a shorter period. In addition, children received fewer hours per week of therapy than in previous reports. Children in the experimental treatment group were pairwise matched to children in a control group (who received conventional school-based and brief one-on-one interventions) on the basis of pretreatment chronological and mental age, diagnosis (autism vs. PDD), and length of treatment. The groups did not differ on pretreatment IQ. Children receiving the experimental treatment had significantly higher posttreatment IQ scores. Smaller, but still statistically significant effects on symptom severity were also found, though experimental subjects still met diagnostic criteria for autism or PDD.