Background: A history of separation anxiety disorder (SAD) is frequently reported by patients with obsessive–compulsive disorder (OCD). The purpose of this study was to determine if there are clinical differences between OCD-affected individuals with, versus without, a history of SAD. Methods: Using data collected during the OCD Collaborative Genetic Study, we studied 470 adult OCD participants; 80 had a history of SAD, whereas 390 did not. These two groups were compared as to onset and severity of OCD, lifetime prevalence of Axis I disorders, and number of personality disorder traits. Results: OCD participants with a history of SAD were significantly younger than the non-SAD group (mean, 34.2 versus 42.2 years; P<.001). They had an earlier age of onset of OCD symptoms (mean, 8.0 versus 10.5 years; P<.003) and more severe OCD, as measured by the Yale–Brown Obsessive Compulsive Scale (mean, 27.5 versus 25.0; P<.005). In addition, those with a history of SAD had a significantly greater lifetime prevalence of agoraphobia (odds ratio (OR) = 2.52, 95% confidence interval (CI) = 1.4–4.6, P<.003), panic disorder (OR = 1.84, CI = 1.03–3.3 P<.04), social phobia (OR = 1.69, CI 1.01–2.8, P<.048), after adjusting for age at interview, age at onset of OCD, and OCD severity in logistic regression models. There was a strong relationship between the number of dependent personality disorder traits and SAD (adjusted OR = 1.42, CI = 1.2–1.6, P<.001). Conclusions: A history of SAD is associated with anxiety disorders and dependent personality disorder traits in individuals with OCD. Depression and Anxiety 28:256–262, 2011. © 2011 Wiley-Liss, Inc.