A link between dissociation proneness in adulthood and self-reports of childhood traumatic events (including familial loss in childhood, sexual/physical abuse and neglect) has been documented. Several studies have also provided evidence for an association between dissociative experiences and trauma in patients with various psychiatric disorders, including post-traumatic stress disorder, borderline personality, dissociative identity and eating disorders. Based on the relative paucity of data on dissociation and trauma in obsessive-compulsive disorder (OCD) and trichotillomania (TTM), the primary objective of this study was to examine the relationship between trauma and dissociative experiences (DE) in these two diagnostic groups. Furthermore, the availability of clinical and genetic data on this sample allowed us to explore clinical and genetic factors relevant to this association. A total of 110 OCD and 32 TTM patients were compared with respect to the degree of dissociation (using the Dissociative Experiences Scale [DES]) and childhood trauma (using the Childhood Trauma Questionnaire [CTQ]). Patients were classified on the DES as either "high" (mean DES score >/= 30) or "low" (mean DES score < 30) dissociators. Additional clinical and genetic factors were also explored with chi-square and t tests as appropriate. A total of 15.8% of OCD patients and 18.8% of TTM patients were high dissociators. OCD and TTM groups were comparable on DES and CTQ total scores, and in both OCD and TTM groups, significant positive correlations were found between mean DES scores and mean CTQ subscores of emotional abuse, physical abuse, sexual abuse, and physical neglect. In the OCD group, high dissociators were significantly younger than low dissociators, and significantly more high dissociators than low dissociators reported a lifetime (current and past) history of tics (P <.001), Tourette's syndrome (P =.019), bulimia nervosa (P =.003), and borderline personality disorder (P =.027). In the TTM group, significantly more high dissociators than low dissociators reported (lifetime) kleptomania (P =.005) and depersonalisation disorder (P =.005). In the Caucasian OCD patients (n = 114), investigation of genetic polymorphisms involved in monoamine function revealed no significant differences between high and low dissociator groups. This study demonstrates a link between childhood trauma and DE in patients with OCD and TTM. High dissociative symptomatology may be present in a substantial proportion of patients diagnosed with these disorders. High dissociators may also be differentiated from low dissociators on some demographic features (e.g., lower age) and comorbidity profile (e.g., increased incidence of impulse dyscontrol disorders). Additional work is necessary before conclusions about the role of monoaminergic systems in mediating such dissociation can be drawn.