The paper describes a feasibility study of the 'tracking project', a new method for examining health and social care agency overlap, repeat service use and shared patients. It demonstrates that this method is a practical and effective means of informing policy. The Tracking Project is the first project in this country to combine total population databases from a range of social, health care and criminal justice agencies. This has enabled, for the first time, the development of a multi-agency database for total health, mental health, social care and criminal justice populations in one county for a 2-year period, (number of cases = 97,162) through standardised anonymisation of agency databases, using SOUNDEX a software programme. The Tracking method has potential for developing screening instruments, through post-sample testing of predictive variables, in order to enable identification of multiple agency attenders for targeting or multi-disciplinary working. The authors discuss these data in the context of previous research evidence and current policy, examining reasons why particular clients might use many social and health care agencies and considering the potential of this method to inform service planning and implementation.