HIV transmission can occur when drug users share contaminated needles, syringes, or other equipment. Noninjection drug use is also a source of AIDS risk. People who have sex while under the influence of alcohol or other drugs are more likely to engage in unsafe sex because of impaired judgment or lowered inhibitions. Unsafe sex is reportedly widespread among people who smoke crack cocaine. Recent trends suggest the possibility of a slight increase in seroprevalence among injection drug users in Los Angeles. This trend, however, may be due to variability in the degree to which drug users at highest risk are motivated to seek antibody testing. In several important respects, Los Angeles drug users appear to have reduced their risk of AIDS. Studies conducted from 1986 to 1990 indicate trends toward less frequent sharing of injection equipment and less frequent use of drugs at shooting galleries. Among drug users who still share injection equipment, bleach use has increased. Further, the use of condoms has increased among drug users with multiple sex partners. Favorable trends aside, many injection drug users continue to incur AIDS risk. Most still share injection equipment, and about half do not use bleach. Despite their self-reported risks, most injection drug users still believe that the risk of contracting AIDS is no higher for them than for other people. Limited evidence indicates a high risk of AIDS among incarcerated and homeless youth who report inconsistent condom use, frequent sex while under the influence of alcohol or drugs, and frequent acts of "survival sex" (for money, food, and shelter). Appendixes provide supplemental information on the study's data sources and sample composition. 139 references, 13 tables, and 8 figures