Studies designed to evaluate the services of a health or welfare institution face the major difficulty of selecting evaluative criteria to serve as measures of successful performance. This paper explores that difficulty in the context of a study of a new psychogeriatric day hospital. The study and its methods of data collection are described. ‘Patient turnover’ features prominently as a measure of success within the hospital. Consultants, nurses, general practitioners, social workers, staff of a related hospital and patients’ relatives interpret this measure in different ways and adopt different strategies to pursue ‘success’ in their own interests. These differences are described. The paper concludes that a ‘pluralistic evaluation’ has several advantages as compared to other approaches.