Safety and Effectiveness of D-Cycloserine in Children With Autism

Inclusion Criteria: * Age 3 Years to 12 Years * Diagnostic Statistical Manual Version -IV (DSM-IV) and Autism Diagnostic Interview - Revised (ADI-R)-confirmed Diagnosis of Autistic Disorder * Aberrant Behavior Checklist (ABC) Lethargy Subscale Score of 13 or greater Exclusion Criteria: * Children with Severe to Profound Mental Retardation * Weight at Screening Visit \<11 kilograms * Clinical Global Impressions-Severity Score of 7 * Presence of a Neurodevelopmental Disorder with Possible Associations to Autism: Subjects with Fragile X Syndrome, Tuberous Sclerosis, or other neurodevelopmental disorders known to be associated with autism or autistic features will be excluded. * Presence of a Psychiatric Disorder that would Require a Specific Type of Treatment: Subjects with major depressive disorder, bipolar disorder, or a psychotic disorder will be excluded because treatment for these disorders often requires specific psychotropic agents. Subjects with an active substance use disorder will be excluded because of safety concerns and problems this would cause in assessing efficacy. * Presence of a Medical Condition that would make Treatment with D-Cycloserine Less Safe: Subjects with significant cardiac, hepatic, or renal disease will be excluded due to concerns about pharmacokinetic alterations or adverse effects. Subjects with epilepsy or a history of seizures will be excluded due to rare reports of seizures with high doses of D-cycloserine. D-cycloserine is an U.S. FDA Pregnancy Category C drug. Because of the unknown effects of D-cycloserine on the developing human fetus, females of childbearing potential will be given a urine pregnancy test and required to use a suitable form of birth control during the study.