To evaluate HIV testing behavior, validity of self-reported serostatus, and intention to test among alcoholics and drug users entering treatment. Longitudinal cohort study. A total of 952 clients voluntarily entering three outpatient and two inpatient public alcohol treatment centers in San Francisco were enrolled. Seventy-six per cent were men, 50% black, 81% had used both alcohol and drugs during the last year, 43% had injected drugs and 9% of the men were homosexual. Subjects completed an interviewer-administered questionnaire and blindly-linked HIV-antibody test at entry and after 1 year (81% follow-up). Fifty-seven per cent of subjects reported that they had previously sought HIV testing. Factors associated with HIV testing included homosexual contact, injecting drug use, having a partner who had been tested, and using condoms. Hispanics were the least likely of all ethnic groups to report testing. Of 60 subjects with HIV antibodies, 47 (78%) said they had already been tested; however, 19 (40%) inaccurately reported that their serostatus was negative and another four (9%) had not collected their test results. Blacks were much more likely than other groups to misreport or be unaware of their HIV status. Only half of the 68% who said they planned to be tested during the following year did so. Five (42%) out of 12 HIV-positive individuals who were unaware of, or misreported their serostatus at baseline, and who sought another HIV test during the follow-up year continued to report themselves as uninfected. A large proportion of clients attending public alcohol treatment centers report having been HIV tested, much greater than that observed in other populations. However, misreporting of HIV test results was very common among seropositive subjects. Alcohol and drug treatment programs for this high-risk population should include interventions to optimize use of HIV testing for prevention and treatment, and improve understanding of test results.