Background Obsessive‐compulsive disorder (OCD) is a chronic condition that often produces lifelong morbidity, but few studies have examined long‐term outcome (greater than 5 years) in adult patients. Available studies suggest that 32–74% of adult OCD patients will experience clinical improvement over the long term. However, these studies were conducted before validated OCD rating scales were established and the development of evidence‐based treatments for OCD. Methods We investigated the 10–20 year outcome of 83 of 165 eligible subjects previously enrolled after participation in placebo‐controlled trials of serotonin reuptake inhibitor (SRI) medications for OCD. We examined the association between clinical characteristics at initial assessment and OCD symptom severity at follow‐up. We hypothesized that primary OCD symptom dimension and initial response to pharmacotherapy with serotonin reuptake inhibitors would be associated with later symptom severity. Results Only 20% (17 of 83) of subjects had experienced a remission of their OCD symptoms at follow‐up (Y‐BOCS ≤ 8). Forty‐nine percent (41 of 83) of subjects were still experiencing clinically significant OCD symptoms. Response to initial SRI pharmacotherapy was significantly associated with long‐term outcome: 31% (13 of 42) of subjects who responded (CGI < 3) to initial SRI pharmacotherapy were remitted at follow‐up, compared to 12% (3 of 25) of partial responders and none of the 16 subjects who had no response to initial SRI pharmacotherapy. We did not find a significant association between long‐term clinical outcome and any of the OCD symptom dimensions. Conclusion Despite the introduction and dissemination of several evidence‐based treatments for OCD, most adult OCD patients do not achieve remission. Initial response to pharmacotherapy was strongly associated with long‐term outcome.