The main objective of this study is to evaluate the safety and effectiveness of memantine (Namenda®) for cognitive and behavioral impairment in adults ages 18-50 years with autism spectrum disorders (ASD). This is an exploratory, 12-week, pilot study, seeking to determine whether Namenda is efficacious and well tolerated in the treatment of adults with ASD. The study results will be used to generate hypotheses for a larger randomized controlled clinical trial with explicit hypotheses and sufficient statistical power.