A Study of Memantine Hydrochloride (Namenda®) for Cognitive and Behavioral Impairment in Adults With Autism Spectrum Disorders

Inclusions * Male and female outpatients 18-50 years of age. * Participants must have DSM-IV-TR diagnosis of PDD and displaying PDD symptoms with at least moderate impairment (SRS score ≥ 85 and CGI-PDD ≥ 4). * Fulfills diagnosis of autism spectrum disorders by meeting DSM-IV-TR PDD diagnostic criteria of autistic disorder (with the exception of a total lack of spoken language), Asperger's disorder, or PDD-NOS as established by clinical interview and confirmed by DICA-R PDD module. * Subjects and/or their legal representative must have a level of understanding sufficient to communicate intelligently with the investigator and study coordinator, and to cooperate with all tests and examinations required by the protocol. * Subjects and/or their legal representative must be considered reliable reporters. * Each subject and/or their authorized legal representative must understand the nature of the study. The subject and/or their legal representative must sign an informed consent document. * Subject must be able to participate in mandatory blood draws. * Subject must be able to swallow pills. * Subjects with mood, anxiety, or disruptive behavior disorders will be allowed to participate in the study provided they do not meet any exclusionary criteria. Exclusions * IQ \< 85. * Total lack of spoken language. * DSM-IV-TR PDD diagnoses of Rett's disorder, or childhood disintegrative disorder. * Clinically unstable psychiatric conditions or judged to be at serious suicidal risk. * Active symptoms of anorexia or bulimia nervosa * Current diagnosis of a psychotic disorder or unstable bipolar disorder. * History of recent or current (past 30 days) clinically significant depressive or anxiety disorder that warrants treatment. * Current diagnosis of schizophrenia. * History of substance use (except nicotine or caffeine) within past 3 months * Serious, stable or unstable systemic illness including hepatic, renal, gastroenterological, respiratory, cardiovascular (including ischemic heart disease), endocrinologic, neurologic, immunologic, or hematologic disease. * Subjects with severe hepatic impairment (LFTs \> 3 times ULN) and those with severely impaired renal function (eGFR \< 30). * Subjects with genitourinary conditions that raise urine pH (e.g., renal tubular acidosis, severe infection of the urinary tract). * Uncorrected hypothyroidism or hyperthyroidism. * Subjects with untreated and/or unstable diabetes. * Non-febrile seizures without a clear and resolved etiology. * Pregnant or nursing females. * Known hypersensitivity to memantine. * Severe allergies or multiple adverse drug reactions. * A non-responder or history of intolerance to memantine, after treatment at adequate doses as determined by the clinician. * Investigator and his/her immediate family defined as the investigator's spouse, parent, child, grandparent, or grandchild.