In response to a high incidence of AIDS among injecting drug users in Barcelona, Spain, the city established a program in four geographically distinct centers for assistance and surveillance. The centers provided testing, counseling, and monitoring of infectious diseases. In 1991-92, HIV rates were 33% in the northwest center, 39% in the western center, 39% in the east, and 71% in the south. Differences between the rates in the southern center when compared with those in the other centers were very significant (P < or = .001). These data show geographic variation of HIV infection within one city and substantiate the need to provide expanded medical care in the drug misuse treatment setting.