The purpose of this study is to test the effect of a moderate-intensity aerobic exercise intervention for Obsessive Compulsive Disorder (OCD) patients interested in reducing symptoms. The investigators expect that this project will contribute much needed knowledge about the role that aerobic exercise can play in managing the effects of OCD. If moderate-intensity aerobic exercise is efficacious in helping individuals with OCD manage obsessions and compulsions, this will establish that aerobic exercise may be a valuable adjunct to other OCD treatments such as medication and therapy.

Inclusion Criteria:

1. are between 18 and 65 years of age
2. are sedentary, i.e. have not participated in regular aerobic physical exercise for at least 90 minutes per week during the past three months
3. currently engaged in at least 12 weeks of stable dose pharmacotherapy and/or are currently engaged in cognitive-behavioral therapy for the past 13 weeks
4. meet Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) criteria for OCD as assessed by the Structured Clinical Interview for DSM Disorders, Patient Version (SCID-P)
5. have Y-BOCS score greater than 16

Exclusion Criteria:

1. DSM-IV diagnosis of substance abuse/dependence except for nicotine dependence
2. DSM-IV diagnosis of anorexia or bulimia nervosa
3. DSM-IV diagnosis of bipolar disorder
4. history of psychotic disorder or current psychotic symptoms
5. current suicidality or homicidality
6. marked organic impairment
7. physical or medical problems that would not allow for safe exercise participation
8. current pregnancy or intent to become pregnant during the next 12 weeks