Memantine is a glutamate receptor antagonist that has been reported to reduce Obsessive-Compulsive Disorder (OCD) symptoms in case studies of treatment-resistant individuals. The investigators hypothesized that memantine is an effective augmenting agent to standard intensive residential treatment of severe OCD. An intent-to-treat, single-blinded, naturalistic case-control design is employed. The sample includes subjects receiving standard treatment at the McLean/ MGH Intensive Residential Treatment (IRT) program, half of whom also receive memantine augmentation. Admission, monthly and discharge measures of OCD, depression and psychosocial functioning are collected by raters blinded to augmentation status. Matched controls are selected based upon gender, initial OCD severity, psychosocial functioning, and timing of admission. Descriptive and comparative analyses are conducted via SPSS, statistical significance is defined at p<0.05, clinically significant response is defined by a 25% reduction, and 'marked response' is defined by a 50% improvement in Yale-Brown Obsessive Compulsive severity (Y-BOCS) scores, using a last-observation-carried-forward approach. The Clinical Global Improvement (CGI) scale captures global clinical change.