Objective Empirical research on the negative effects of psychotherapy is insufficient, partly because there is a lack of theoretical concept on how to define, classify and assess psychotherapy side effects. This paper proposes a model for the definition, classification and assessment of psychotherapy side effects. Method Definitions are provided for ‘unwanted events’, ‘treatment‐emergent reactions’, ‘adverse treatment reactions’, ‘malpractice reactions’, ‘treatment non‐response’, ‘deterioration of illness’, ‘therapeutic risks’ and ‘contraindications’. We describe processes analysing the causality of negative events and the correctness of treatment. Result A procedural model on how to find, classify and evaluate negative events is described, the unwanted event to adverse treatment reaction (UE–ATR) checklist. Conclusion Recognition of adverse treatment effects is a characteristic of good therapists and treatments. Psychotherapists should be sensitive for negative effects. This can help improve the quality of treatment. The UE–ATR checklist can be used in psychotherapy trials, quality assurance, clinical practice and training of psychotherapists.Copyright © 2012 John Wiley & Sons, Ltd. Key Practitioner Message If you do not find adverse treatment effects, then ask yourself why and do not assume that there are no side effects.The detection and management of adverse treatment effects is not a sign of bad but of good clinical practice.