BACKGROUND HIV counseling prevents sexually transmitted diseases (STDs), with most of the benefit accumulating in the first 6 months. STUDY The authors conducted a multicenter, randomized, controlled trial of a 20-minute additional (booster) counseling session 6 months after HIV counseling compared with no additional counseling for prevention of STDs (gonorrhea, chlamydia, trichomoniasis). Participants were 15- to 39-year-old STD clinic patients in Denver, Long Beach, and Newark. RESULTS Booster counseling was completed by 1120 (67.8%) of 1653 assigned to receive it. An incident STD during the 6 to 12 months after initial counseling (and within the 6 months after scheduled booster counseling) was detected in 141 of 1653 (8.5%) participants in the booster counseling group and 144 of 1644 (8.8%) in the no-booster group (relative risk, 0.97; 95% confidence interval, 0.78-1.22). Three months after booster counseling, sexual risk behaviors were reported less frequently by the booster group than the no-booster group. CONCLUSIONS Booster counseling 6 months after HIV testing and counseling reduced reported sexual risk behavior but did not prevent STDs.