The sharing of needles for injecting drugs is the primary method of transmitting HIV virus among IV drug users. The AIDS epidemic in IV drug users is centered in the New York City metropolitan area, but has started in other cities with large numbers of IV drug users. 1984 studies of HIV seropositivity among IV drug users in New York City showed approximately 60 percent of the IV drug users tested had been exposed to the virus, which is also well-established among IV drug users in continental Europe. In the United States, IV drug users form a link to two other groups at increased risk for AIDS: heterosexual partners and children. There is consistent evidence from a variety of sources to show that IV drug users in New York City have changed their behavior in order to reduce the chances of exposure to the AIDS virus. A great majority (approximately 90 percent) knew that it was spread through sharing needles and approximately 60 percent reported efforts to reduce the chances of exposure, primarily through increased use of sterile needles. Currently, a group of over 300 IV drug users, approximately 60 percent of whom have been exposed to HIV, are being followed to monitor the early course of HIV infection as well as to determine the various possible outcomes. This followup is also being used to examine possible cofactors, such as use of noninjected drugs and stress, that might influence the progression of HIV infection, which may have great potential for reducing the adverse consequences of viral exposure. Therapeutic community experiences indicate that spread of the virus is generally well ahead of the development of strategies for coping with the difficult problems associated with AIDS. 15 references.