Objective. —To evaluate the effectiveness of human immunodeficiency virus (HIV) testing and posttest counseling in reducing subsequent high-risk behavior. Methods. —The incidence of sexually transmitted diseases (STDs) in the Baltimore, Md, public STD clinic population after HIV testing and counseling was determined by chart review and was compared in two groups, 868 HIV-seropositive patients and 1104 HIV-seronegative patients, matched by age, sex, and month in which HIV test was conducted. Patients were observed for incident STDs at intervals of 6 through 23 months. Patients with incident STDs were classified hierarchically after being notified of the HIV test result and receiving posttest counseling. Results. —Of HIV-seropositive patients, 615 (71%) returned for their test results and received posttest counseling; 694 HIV-seronegative patients (63%) returned. Of all those who returned for results and posttest counseling, 60 (9.7%) of 615 HIV-seropositive patients and 61 (8.8%) of 694 HIV-seronegative patients were diagnosed at least once with definite STD (syphilis, gonorrhea, or trichomoniasis) (P, not significant). Twenty-four HIV-seropositive patients (3.9%) and 71 HIVseronegative patients (10.2%) returned with probable STD (nongonococcal urethritis or pelvic inflammatory disease) (P<.001). Nine HIV-seropositive patients (1.5%) and 23 HIV-seronegative patients (3.3%) returned having had an STD-infected sexual partner (P<.03). Age, sexual orientation, and drug use behavior did not predict return with STD. Conclusions. —Both HIV-seropositive and HIV-seronegative patients showed high rates of repeat STDs after posttest counseling, an important public health challenge in creating effective high-risk behavior prevention strategies. © 1992, American Medical Association. All rights reserved.