Antibodies against glutamic acid decarboxylase 65 (GAD65) have been detected in the serum of patients with several neurological disorders. The presence of antibodies against GAD65 has not yet been examined in the serum of patients with neurodevelopmental disorders such as autism or attention-deficit/hyperactivity disorder (ADHD). In this study, GAD65 antibodies and total IgG were assayed in the serum of normal subjects and patients diagnosed with autism or ADHD. GAD65 antibodies were detected in the serum of 15% of children with autism (N = 20), 27% of children with ADHD (N = 15) and of none of the controls (N = 14). The serum of 60% of autistic and 53% of ADHD patients reacted with Purkinje neurons in mouse cerebellum. Serum from 20% of ADHD patients reacted also with the cells in the molecular and granule cell layers and cells in the vicinity of the Purkinje neurons. No association was found between the titer of GAD65 antibodies and total IgG levels, and presence of seizures or mental retardation. None of the ADHD patients were diagnosed with mental retardation. Serum anti-GAD65 antibodies may be a common marker of subgroups of patients with autism and ADHD. Reactions of serum antibodies with the cells in the cerebellum in these patients suggest direct effects on brain function. The subgroup of children with autism and ADHD that tests positive for GAD65 antibodies needs further characterization in a larger study.