Exposure and Response Prevention Therapy for Obsessive-compulsive Disorder

Inclusion Criteria: * Minimum score of \>16 on the Y-BOCS; this score indicates clinically significant OCD symptom severity; * Medically healthy * Outpatient men and women age 18 years and older; * Meets DSM-IV criteria for a diagnosis of OCD as determined by the MINI. OCD is the primary psychiatric diagnosis (i.e., it is defined by the patient as the most important source of current distress); * Able to communicate meaningfully with the investigators and competent to provide written informed consent. IQ estimate \> 85 standard score. Exclusion Criteria: * Current clinically significant suicidality or individuals who have engaged in suicidal behaviors within the past 12 months will be excluded and referred for appropriate clinical intervention; * Presence of any clinical features requiring a higher level of care (inpatient or partial hospital treatment); * Current or past DSM-IV bipolar disorder (Current or past unipolar depression is not an exclusion criterion. We will allow comorbid depression, to ensure that we are treating a representative OCD sample. Depression is the most common complication in OCD, and about one third of the OCD clinical samples are depressed.) * Schizophrenia, schizoaffective disorder, or any other current or lifetime DSM-IV psychotic disorder; * Current or recent (within 3 months of study entry) DSM-V alcohol or drug dependence or abuse (other than nicotine), or a positive urine drug screen for any illicit substances of abuse; * Estimated IQ \<85 on the WASI, mental retardation, dementia, brain damage, or other cognitive impairment that would interfere with the capacity to participate in the study and complete self-report measures; * Any medical conditions that might contraindicate use of the study treatments, as determined by medical physical and laboratory tests by the study physicians; * Presence of any significant and/or unstable medical illness which might lead to hospitalization during the study duration; subjects with a stable medical condition may participate with the agreement of the subject's physician and the study physician who performs the study physical exam; * Concurrent psychotherapy of any type (e.g., CBT/ERP, supportive, psychodynamic) and duration; * History of adequately-delivered exposure-based cognitive-behavioral therapy (i.e., 10 or more sessions of specific and regular exposure and/or response prevention assignments; as needed and with patients' written permission we will confer with prior treatment providers).