Abstract: The study was commissioned to define the role of Advanced Nurses Practitioner to inform the establishment of a clinical career pathway. Four new, loosely defined ANP positions were created and the role evolution over three months was described. The impact on the multidisciplinary team functioning was to be given particular attention. The research design was developed in collaboration with the ANP Project Team of the service. Data were derived from surveys of nurses in the units and other staff before and at the end of the 3 months period; interviews with the ANPs and official client advocates; daily journals and weekly logs kept by the ANPs; statistical records of patient loads and staffing. The findings presented the role as the interface of unit management and direct client care, with the ANPs orchestrating the activities of the unit. The ANPs developed the role differently according to quite distinct conceptualisations of nursing which influenced whether direct client care was pivotal or peripheral to the role. This had an effect on whether the strains of the service were seen as inhibitory of focal to the development of the ANP practice. There was little change in unit staff satisfaction. Attempts to incorporate client advocacy to determine change in client satisfaction were unsuccessful. The ANPs used the research as a process of role development