An epidemiologic field methodology is described for researching and intervening in urban heroin spread. Field concepts are based upon the size of the disease outbreak in definable neighborhoods: the isolated case, the microepidemic, and the macroepidemic. Field observations and Illinois Drug Abuse Program admission data are presented to illustrate the utility of these concepts. Our data suggest that heroin spreads through existing drug using friendship groups and that "contagious" individuals tend to be in the early stages of heroin experimentation or addiction. Our approach for intervening in heroin spread is based upon established public health principles used to halt epidemics of certain contagious diseases, namely, early identification of new outbreaks and involvement of all diseased persons in treatment to prevent them from spreading the disorder to others. This intervention strategy was used to contact and involve in treatment the majority of new addicts produced by a microepidemic in one Chicago neighborhood.