Growing attention has been paid to negative effects of psychotherapeutic interventions. Yet not much is known about their relationship to treatment outcome. Do they pose an obstacle to successful treatment, or is some level of inconvenience necessary to get better? The aim of this study is to assess the prevalence of the negative effects, as well as their relationship to outcome variables, in group psychotherapy setting. The sample consists of 225 Czech clients suffering from common mental disorders. Data on the therapeutic process and outcome have been collected at seven clinical sites and negative effects were assessed by the Negative Effects Questionnaire (NEQ) at treatment completion. The analysis will be focused on the relationships between several areas of negative effects (as defined by the NEQ dimensions) and outcome variables (depression, anxiety, well-being, and somatic symptoms).