Certain childhood neuropsychiatric disorders, including Sydenham's chorea Obsessive Compulsive Disorder (OCD) and Tourette's disorder, appear to be mediated (for a subgroup of these patients) through antibodies produced against Group A beta-hemolytic streptococcal infections which cross-react with neurons within the basal ganglia and other regions. This has led to the identification of a clear subtype of patients with OCD and Tourette's Disorder, who appear to distinguishable from other forms of OCD and movement disorders. This subtype is identified by acronym: PANDAS (Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptoccal infections). Volumetric MRI Examinations confirm basal ganglia involvement in Sydenham's chorea, while the preliminary success of immunomodulatory treatments, such as plasma exchange and intravenous immunoglobulin, suggest that the antibodies play an etiologic role. Ongoing treatment studies for this include immunomodulatory interventions (i.e. plasmapheresis, intravenous immunoglobulins) for Sydenham's Chorea and "antineuronal mediated" OCD and tic disorders. Additionally, a double-blind controlled trial of penicillin versus placebo is underway in a group of children with OCD ( or tic disorders) to determine if they have fewer and less severe exacerbations during the penicillin treatment. Examinations of CSF obtained at follow-up of children wit OCD who had participated in the clomipramine/desipramine crossover investigation have shown "normalization" of monoamines and neuroendocrine measures during clomopramine maintenance therapy. Of particular interest, somatostatin concentrations decreased and the growth rate of the affected children appeared to have increased. Longitudinal data are currently being examined from a subgroup of these children. Further, post-treatment PET scans were obtained from a cohort of patients with OCD to determine changes in patterns of cerebral blood flow following treatment.