Obsessive-compulsive disorder (OCD) is a heterogeneous and disabling condition; however, no studies have examined symptom categories or subtypes as predictors of long-term clinical course in adults with primary OCD. A total of 213 adults with DSM-IV OCD were recruited from several mental health treatment sites between July 2001 and February 2006 as part of the Brown Longitudinal Obsessive Compulsive Study, a prospective, naturalistic study of treatment-seeking adults with primary OCD. OCD symptoms were assessed annually over the 5-year follow-up period using the Longitudinal Interval Follow-Up Evaluation. Thirty-nine percent of participants experienced either a partial (22.1%) or a full (16.9%) remission. Two OCD symptom dimensions impacted remission. Participants with primary obsessions regarding overresponsibility for harm were nearly twice as likely to experience a remission (P < .05), whereas only 2 of 21 participants (9.5%) with primary hoarding achieved remission. Other predictors of increased remission were lower OCD severity (P < .0001) and shorter duration of illness (P < .0001). Fifty-nine percent of participants who remitted subsequently relapsed. Participants with obsessive-compulsive personality disorder were more than twice as likely to relapse (P < .005). Participants were also particularly vulnerable to relapse if they experienced partial remission versus full remission (70% vs 45%; P < .05). The contributions of OCD symptom categories and comorbid obsessive-compulsive personality disorder are critically important to advancing our understanding of the prognosis and ultimately the successful treatment of OCD. Longer duration of illness was also found to be a significant predictor of course, highlighting the critical importance of early detection and treatment of OCD. Furthermore, having full remission as a treatment target is an important consideration for the prevention of relapse in this disorder.