Objective: The aim of this study was to investigate the interrelationships between childhood trauma, attachment, alexithymia, and the severity of obsessive compulsive disorder (OCD) in a cohort of participants with OCD. Rationale: There is a growing body of research linking traumatic experiences in childhood with the development of OCD. The mechanisms involved in this association are not yet clear. Methods: The sample was comprised of 82 people with OCD and 92 comparison participants. A cross-sectional design was used, utilizing internet-mediated administration of the Childhood Trauma Questionnaire - revised (CTQ-R); the Yale-Brown Obsessive Compulsive Scale - Self-Report (Y-BOCS-SR); the Experiences in Close Relationships Scale (ECR); and Toronto Alexithymia Scale (TAS-20). Partial least squares (PLS) analysis was used to determine significant paths between the constructs. Results: Results of PLS analysis supported all of the hypotheses made: there was a significant positive correlation between childhood trauma and attachment avoidance, which in turn was significantly positively associated with alexithymia. Alexithymia was significantly associated with the severity of OCD symptoms and the number of OCD symptoms. Mediational analysis showed that alexithymia significantly carried an influence from attachment avoidance to the severity of obsessions and the number of obsession symptoms. Conclusions: There is a relationship between childhood trauma and OCD, however this relationship is not direct in nature but is influenced by peoples' past experiences with significant others and associated difficulties in emotional processing. © 2011 The British Psychological Society.