This paper examines some of the data obtained from the Joint Board of Clinical Nursing Studies Course Number 650, which ran at the Bethlem Royal and Maudsley Hospitals, London, 1978–1979. During this particular training programme, eight nurse therapist trainees treated a total of 251 patients assessed as suitable for behaviour therapy. Data were collected from patients by trainees on four separate occasions; before treatment, after treatment, and at follow-up intervals of 1 and 6 months. It was suggested that therapists would vary systematically in terms of the assessment scores used to measure outcome. This was explored using analysis of variance techniques, where several treatment outcome measures were used as dependent variables. The analyses, which were undertaken using SPSS and GLIM computer packages, clearly demonstrated therapist variability. The results are discussed within the theoretical framework of Sudman & Brabum's (1974) interviewing model and O'Muircheartaigh & Wiggins' (1981) consideration of response errors. The implications for the selection and training of nurse therapists are presented. The final conclusion of the paper is that, although the patient's clinical outcome may be related to therapist allocation, the eight trainees allowed themselves to ‘open’ their activities to this evaluative approach – which in turn demonstrates their professionalism.