Objective. We set out to investigate the association between negative trauma-related cognitions, emotional regulation strategies, and attachment style and complex posttraumatic stress disorder (CPTSD). As the evidence regarding the treatment of CPTSD is emerging, investigating psychological factors that are associated with CPTSD can inform the adaptation or the development of effective interventions for CPTSD. Method. A cross-sectional design was employed. Measures of CPTSD, negative traumarelated cognitions, emotion regulation strategies, and attachment style were completed by a British clinical sample of trauma-exposed patients (N = 171). Logistic regression analysis was used to assess the predictive utility of these psychological factors on diagnosis of CPTSD as compared to PTSD. Results. It was found that the most important factor in the diagnosis of CPTSD was negative trauma-related cognitions about the self, followed by attachment anxiety, and expressive suppression.