Seroprevalence among injection drug users did not increase in 1991, and seroconversion was extremely low. It appears that the rate of HIV infection is no higher than 3 percent for injection opiate users in methadone maintenance/detoxification treatment and no higher than 7 percent among injection drug users not in treatment. With treatment and nontreatment estimates weighted by the likely proportion of users in treatment, the overall rate of HIV infection among Los Angeles injection drug users is estimated at 6 percent. Infection rates, however, may be higher in some populations such as the homeless. A majority of drug users who share needles now report using bleach as a disinfectant. Trends also show a significant increase in the percentage of users who report recent attempts to stop sharing needles. Although a majority of injection drug users who are nonmonogamous now use condoms, there has been no change in the percentage of users who report sex with multiple partners. Cocaine injection does not appear to be associated with higher risk than opiates. Los Angeles arrestees who smoke crack have more sex partners than arrestees who do not smoke crack. This relationship is stronger for women than for men, but is significant in both. Recommendations are offered that concern monitoring HIV infection, reducing risk behavior, and targeting outreach and treatment programs. 67 references, 11 tables, and 5 figures