Obsessive-compulsive disorder is a common, chronic, and oftentimes disabling disorder. The only established first-line treatments for obsessive-compulsive disorder are exposure and response prevention therapy and the serotonin reuptake inhibitors. Many patients do not experience complete symptom resolution with either modality and require augmentation approaches. Recent animal and clinical data suggest that D-cycloserine, a partial agonist that acts at the strychnine-insensitive glycine-recognition site of the N-methyl-D-aspartate receptor complex, may enhance extinction learning that occurs in exposure-based psychotherapies. Given this, this study examined if D-cycloserine (250 mg) enhances the overall efficacy and rate of change of exposure and response prevention therapy for adult obsessive-compulsive disorder. Participants were 24 adults meeting Diagnostic and Statistical Manual of Mental Disorders-IV criteria for obsessive-compulsive disorder. The study design was a randomized, double-blinded, placebo-controlled augmentation trial examining exposure and response prevention therapy+D-cycloserine versus exposure and response prevention therapy+placebo. All patients received 12 weekly sessions of exposure and response prevention treatment. The first session involved building a ritual hierarchy and providing psychoeducation about obsessive-compulsive disorder. The second session involved a practice exposure. Sessions 3-12 involved exposure and response prevention exercises. D-cycloserine or placebo (250 mg) was taken 4 h before every session. No significant group differences were found across outcome variables. The rate of improvement did not differ between groups. The present results fail to support the use of D-cycloserine with exposure and response prevention therapy for adult obsessive-compulsive disorder. As this study is the first to explore this question and a number of methodological issues must be considered when interpreting the findings, the conclusions that may be drawn from our results are limited.