Background: The present study examined sudden gains during treatment for obsessive-compulsive disorder (OCD) and their relationship to short-and long-term outcome. Methods: Ninety-one individuals (age 19-64) completed either cognitive treatment, exposure treatment, or their combination with fluvoxamine for OCD. Participants' obsessive-compulsive symptoms were assessed before each weekly treatment session. In addition, obsessive-compulsive and depressive symptoms were assessed pre treatment and post treatment as well as 12 months following treatment termination. Results: Sudden gains were found among 34.1% of participants and constituted 65.5% of the total reduction in obsessive-compulsive symptoms. Compared to individuals who did not experience sudden gains, individuals who experienced sudden gains reported lower levels of OCD symptoms post treatment, and this was maintained during follow-up. Conclusions: Sudden gains are common in treatments for OCD and are predictive of treatment outcome and follow-up. Sudden gains mark a distinct trajectory of response to treatment for OCD. Individuals with sudden gains greatly improve during treatment and maintain their gains during follow-up, whereas individuals without sudden gains improve to a significantly lesser extent. Thus, treatment planning and development can benefit from considering sudden gains and the intra-individual course of improvement. © 2011 S. Karger AG, Basel.