The objective of this study was to investigate the interrelationship between chronic idiopathic urticaria (CIU), psychological co-morbidity, posttraumatic stress, repression and alexithymia. 89 participants with CIU and 105 without CIU responded to an online questionnaire. Both groups completed the General Health Questionnaire-12, the Perceived Stress Scale, the Posttraumatic Stress Diagnostic Scale and the Toronto Alexithymia Scale-20 and were categorised into four defence mechanism groups (repressive, defensive, high-anxious, low-anxious). CIU participants also completed the Skindex-17 and a self-report severity measure. CIU participants reported higher levels of alexithymia than the control group and their defence mechanism was most likely to be categorised as defensive, with conscious self-image management reported alongside high manifest anxiety. Partial least squares analysis revealed significant paths between posttraumatic stress and CIU severity and psychological co-morbidity. Posttraumatic stress was associated with alexithymia and type of defence mechanism. Only being in the high-anxious group partially mediated the relationship between posttraumatic stress and CIU severity. In conclusion, there is evidence for a relationship between CIU and trauma. The severity of posttraumatic symptoms varies depending upon alexithymic traits and defence mechanisms used. Disease severity and psychological co-morbidity are differentially influenced by the relationships between trauma, alexithymic traits and defence mechanisms. © Springer Science+Business Media, LLC 2012.