Selective serotonin reuptake inhibitors (SSRIs) are currently the first-line pharmacological agents in treating obsessive-compulsive disorder (OCD). Appropriate treatment for OCD also involves cognitive behavioural therapy (CBT), including exposure and response prevention. As there is a time delay in seeing full therapeutic response, and not all patients tolerate SSRIs, there remains an unmet need for additional treatment approaches in OCD. In addition, most responders report only a partial reduction in symptoms. Clonazepam has demonstrated effectiveness in several preliminary reports in treating OCD. Twenty-seven patients with OCD were entered into a 10-week, double-blind, parallel design trial of clonazepam vs. placebo. Overall, only 3 out of 25 patients who had ≥ 1 rating on clonazepam/placebo were judged to be treatment responders, by scoring a 1 (very much improved) or 2 (much improved) on the CGI improvement scale. Responders included 2 of 9 in the placebo group and 1 of 16 in the clonazepam group. No significant difference was found between clonazepam and placebo groups on responder/non responder status (X² = 1.39, df = 1,24, p=0.238), nor on change in YBOCS, HAM-A, HAM-D or NIMH scales from beginning to last evaluation carried forward. These findings suggest that clonazepam is not effective as monotherapy in treating OCD. Its effectiveness in specific subgroups of OCD patients with co-morbid anxiety disorders or as an augmentation strategy added to SSRIs remains to be determined.