An action research model is presented for the evaluation of community-based interventions. The short- and long-term application of this model is illustrated by means of a study of a community drug team which was initiated to prevent the spread of HIV infection amongst substance misusers in south-west London. A client caseload of 155 was monitored over a year-long period and the results used to evaluate the extent to which the CDT had achieved its stated aims and objectives. Other tools used in the evaluation and monitoring were surveys of generic, specialist and general practitioner services, and needs assessment of samples of drug users. The potential of the model is explored, both in terms of its research role and in facilitating service development. The value of action research methodology in assessing HIV/AIDS prevention is stressed.