Autism may involve an autoimmune pathogenesis in a subgroup of patients. The frequency of anti-nuclear antibodies in 80 autistic children and their relationship to a family history of autoimmunity were studied, compared with 80 healthy, matched children. Children with autism had a significantly higher percent seropositivity of anti-nuclear antibodies (20%) than healthy children (2.5%; P < 0.01). Fifty percent of anti-nuclear antibody-seropositive autistic children had an anti-nuclear antibody titer of > or =1:640 (very high positive); 25%, > or =1:160 (high positive); and the remaining 25%, 1:80. All anti-nuclear antibody-seropositive healthy children had anti-nuclear antibody titers of 1:80. A family history of autoimmunity was significantly higher in autistic children (47.5%) than healthy controls (8.8%; P < 0.001). Anti-nuclear antibody seropositivity was significantly higher in autistic children with a family history of autoimmunity than those without such history (36.8% and 5%, respectively; P < 0.001). Anti-nuclear antibody seropositivity had significant positive associations with disease severity, mental retardation and electroencephalogram abnormalities. Autoimmunity may play a role in a subgroup of children with autism. Further studies are warranted to assess anti-nuclear antibody seropositivity, other markers of autoimmunity (e.g., brain-specific autoantibodies), and the role of immunotherapy in children with autism.