Background: Surprisingly little is known about the long-term course of obsessive-compulsive disorder (OCD). This prospective study presents 2-year course findings, as well as predictors of course, from the Brown Longitudinal Obsessive Compulsive Study, the first comprehensive prospective investigation of the observational course of OCD in a large clinical sample. Method: The sample included 214 treatment-seeking adults with DSM-IV OCD at intake who identified OCD as the most problematic disorder over their lifetime. Subjects were enrolled from 2001-2004. At annual interviews, data on weekly OCD symptom status were obtained using the Longitudinal Interval Follow-Up Evaluation. Probabilities of full remission and partial remission over the first 2 years of collected data and potential predictors of remission were examined. Results: The probability of full remission from OCD was 0.06, and the probability of partial remission was 0.24. Of the 48 subjects whose OCD symptoms partially or fully remitted, only 1 relapsed within the first 2 years. Earlier age at onset of OCD, greater severity of symptoms at intake, older age at intake, and being male were associated with a decreased likelihood of remission. Insight, diagnostic comorbidity, and treatment were not found to be associated with the likelihood of achieving full or partial remission. Conclusions: Though one-quarter of the sample had periods of subclinical OCD symptoms during the prospective period, full remission was rare, consistent with the view of OCD as a chronic and persistent illness. Age at onset, OCD symptom severity, current age, and sex emerged as potent predictors of course. J Clin Psychiatry 2010;71(8):1033-1039 (C) Copyright 2010 Physicians Postgraduate Press, Inc,