OBJECTIVE Previous research has indicated that dissociation might be a negative predictor of treatment outcome in cognitive behavioural therapy for patients with obsessive-compulsive and anxiety disorders. Using a naturalistic design it was hypothesized that higher levels of dissociation predict poorer outcome in inpatients with affective, anxiety and somatoform disorders participating in a brief psychodynamic psychotherapy. METHODS A total of 133 patients completed the Symptom Check List (SCL-90), the German short version of the Dissociative Experiences Scale and the Inventory of Interpersonal Problems at the beginning and the end of treatment. The Global Severity Index (GSI) of the SCL-90 was chosen as outcome criterion. RESULTS A total of 62.4% of study participants were classified as treatment responders, that is, they showed a statistically significant change of their GSI scores. Controlling for general psychopathology, the non-responders had significantly higher baseline dissociation scores than the responders. In a logistic regression analysis with non-response as a dependent variable, a comorbid personality disorder, low baseline psychopathology and high dissociation levels emerged as relevant predictors, but interpersonal problems and other comorbid disorders did not. CONCLUSIONS Dissociation has a negative impact on treatment outcome. It is suggested that dissociative subjects dissociate as a response to negative emotions arising in psychotherapy leading to a less favourable outcome. Additionally, dissociative patients may have an insecure attachment pattern negatively affecting the therapeutic relationship. Thus, dissociation may directly and indirectly influence the treatment process and outcome.