Potential participants will undergo an initial screening visit that will include an interview on psychological symptoms associated with OCD and possible family history of OCD. Eligible participants will then undergo a physical exam, blood draw, DNA sampling, and pregnancy test if applicable. Participants will be randomly assigned to receive either sertraline or placebo daily for 18 weeks. At weekly study visits, participants will receive their study drug, complete questionnaires about symptoms of OCD, and undergo vital sign measurements. At specified visits, participants will also perform a task (Stop Signal Task) on a computerized assessment device to measure attention and impulse control and may have blood drawn. For the first 4 weeks participants will wear a wristwatch-like device (actigraph) to monitor sleep patterns. During the first three visits, participants will receive supportive psychotherapy. At Visit 4, participants will begin receiving 60-minute CBT sessions, which will continue until the final visit. The final visit will include a second physical exam, questionnaires, and blood testing.