Pervasive developmental disorders (PDDs) are life-long childhood neurobiological disorders that cause marked problems with social interaction and communication. In addition to the core impairments in socialization and language, many persons with PDDs often experience high levels of anxiety. Little to no research has been done on treating anxiety symptoms in ASDs, yet preliminary studies suggest it is a common and disabling symptom in many persons. Currently used medications are frequently ineffective or cause troublesome adverse effects. The primary objective of this application is to conduct a preliminary placebo controlled trial of mirtazapine for the treatment of anxiety associated with PDDs. Mirtazapine is novel in that it has both noradrenergic and serotonergic properties. The central hypothesis of this project is that mirtazapine will improve anxiety when administered to children and adolescents with PDD. Thirty children and adolescents (ages 5-17 years) will be randomized to either mirtazapine (n=20) or placebo (n=10) and assessed over 10 weeks of treatment. Finding preliminary evidence of mirtazapine efficacy will be highly significant given the paucity of research involving anxiety in PDD. In addition to improving our understanding of anxiety and its treatment in PDD, results from this trial would be crucial in the development of a larger multi-site clinical trial. When completed, this study will be the first prospective trial of mirtazapine in ASDs, as well as one of the first to examine treatments for anxiety in ASDs. Furthermore, it will give important information about the utility of various outcome measures in measuring anxiety in ASDs.