Before we could carry out systematic studies and intervention projects on the active heroin addict population of our community, we were in need of a field concept that would define addicts as members of a social system rather than as individual social units. Early observations suggested that most heroin users in an urban community are already members of a natural group structure, ie, they participate daily in the operations of a neighborhood heroin-distribution system. To explore the possibility of organizing epidemiologic activities around this drug-distribution system, a field team was assigned to the intensive study of one such copping area. The team observed and gathered information from all addicts who regularly obtained or distributed heroin at this site for a period of six months. This experience suggests that it is possible to obtain epidemiologic data on addicts who frequent a copping area, and that it might be possible to study the impact of treatment, and other competing social factors on the prevalence of active heroin use at such sites.