Obsessive-Compulsive Disorder (OCD) is a common neuropsychiatric illness beginning in childhood. Effective OCD treatments include cognitive-behaviour therapy (CBT) and medications but access to treatment is difficult and does not systematically include parents. The investigators will evaluate clinical and neural effects of Group-based Family CBT (GF-CBT), via a case-control study including: Group 1 - OCD cases receiving GF-CBT (N=90); Group 2 - OCD waitlist cases (N=90). Effects will be measured between baseline and completion of 12 GF-CBT sessions: comparing OCD severity and functioning changes between Groups 1 and 2.

Effective OCD treatment approaches in children and youth include cognitive-behaviour therapy (CBT) and serotonergic medications; however, the disorder is under-diagnosed and access to care is frequently delayed or difficult to obtain. Moreover, despite the important role of family accommodation as an outcome predictor, standard CBT does not systematically include parents and mechanisms of response are not fully understood. The investigators propose to address this challenge by evaluating a novel treatment approach called Group-based Family CBT (GF-CBT).

To determine clinical and neural effects of GF-CBT, we will perform a case-control study of two groups: Group 1 - OCD-affected youth receiving GF-CBT treatment (N=90); Group 2 - OCD-affected youth receiving no new treatment (waitlist controls; N=90).

The investigators will collect clinical outcome data related to OCD severity, individual and family functioning at four time points for Group 1 subjects and at two time points for Group 2 subjects. For Group 1 subjects, these time points include baseline, at midpoint, at the completion of 12 sessions, and at one-month follow-up (all time points are +/- one week). For Group 2 subjects, time points will be at baseline and after a time delay equivalent to completion of 12 GF-CBT sessions. Clinical measures will be used to compare changes between GF-CBT treatment and waitlist-control groups (Groups 1 versus 2).

Inclusion Criteria:

• Male or female, aged 5-18 years old
• Ability of subject and parent to provide informed assent/consent
• English-speaking
• Current DSM IV-TR Axis I diagnosis of childhood-onset OCD; Moderate to severe OCD (CY-BOCS≥16/40)

Exclusion Criteria:

• Current diagnosis of bipolar disorder, psychosis, mental retardation or pervasive developmental disorder (e.g., autism), and current or past substance dependence/abuse