ObjectivesDiagnosis is ubiquitous in Psychiatry, and whilst it does bring benefits; adverse effects of labelling' may also be possible. This study aimed to evaluate experimentally whether clinicians' judgements about a patient with panic disorder were influenced by an inappropriately suggested diagnosis of comorbid borderline personality disorder (BPD). DesignAn experimental design was used to evaluate clinician's judgements when the nature of the information they were given was varied to imply BPD comorbidity. MethodsTwo hundred and sixty-five clinicians watched a video-recorded assessment of a woman describing her experience of uncomplicated panic disorder' and then rated her present problems and likely prognosis. Prior to watching the video recording, participants were randomly allocated to one of three conditions with written information including the following: (1) her personal details and general background; (2) the addition of a behavioural description consistent with BPD; and (3) the further addition of a label' (past BPD diagnosis). ResultsThe BPD label was associated with more negative ratings of the woman's problems and her prognosis than both information alone and a behavioural description of BPD symptoms'. ConclusionsRegardless of potential actuarial value of such diagnoses, it is concluded that clinicians can be overly influenced by past diagnostic labels in the context of an apparent current comorbid problem, although such biases appear to be less likely if a description of the relevant behaviours is used instead. Thus, the label, rather than the behaviour it denotes, may be stigmatizing in mental health professionals. Practitioner points Diagnostic labels can have an inappropriately negative effect on clinicians' judgements not only of treatment variables such as engagement and response but also risk issues and interpersonal effectiveness. Diagnostic labels can have a greater effect on clinicians' judgements than a behavioural description or clinical presentation. Clinicians should therefore be cautious both in the use of diagnostic labels to describe patients and ensure that these are still valid, and also be mindful of the influence that such labels can have on their own clinical judgements and constantly seek to challenge these. Behavioural descriptions of difficulties are less likely to result in such negative judgements and predictions.