 For this study, we will identify 40 children 4 to 18 years old with autism who also have serious behavioral problems. We will then treat them with risperidone. Blood samples will be obtained prior to treatment and at eight weeks of treatment or study exit. At that time, efficacy will be assessed using the Clinical Global Impression-Improvement scale CGI-I and the Irritability subscale of the Aberrant Behavior Checklist ABC. Blood genomic profiles before and after risperidone treatment will be determined using Affymetrix oligonucleotide microarrays combined with RT-PCR. 