About 1% of Americans suffer from obsessive compulsive disorder (OCD), and about 50% of these cases are considered extreme. This research serves to identify the effectiveness of cognitive behavioral therapy (CBT) as treatment for OCD. The authors meta-analyzed ten studies that provided participants' scores on measures of OCD before and after receiving one of five variations of CBT. Each study was coded for the amount of participants, the tests used to measure OCD symptoms, and the age range of participants. The standardized mean differences in scores on OCD measures before and after treatment were computed. A medium effect size was found for group CBT and individual CBT. The studies show that participants scored lower on OCD measures after receiving CBT. This finding suggests that CBT as treatment for OCD is effective for professionals working with people who suffer from OCD.