Nine autistic children and 37 ethnically homogenous controls were enrolled in the study to assess their human leukocyte antigen (HLA) pattern, and eight healthy children were studied to define their peripheral blood cell subsets. We observed a significant decrease in CD4+ naive and an increase in CD4+ memory T cells in autistic children. These differences were significantly more pronounced in the autistic children bearing the HLA A2 and DR11 alleles. These data support the hypothesis that autism could be due to an immune imbalance occurring in genetically predisposed children.