OBJECTIVE To examine trends in sex behaviors and STD prevalence over time among heterosexual STD clinic populations from 3 urban STD clinics in the United States. STUDY DESIGN Cross-sectional analysis comparing baseline data on risk (self-reported) and STDs (laboratory defined) from 2 randomized controlled trials evaluating counseling efficacy conducted about 5 years apart, Project RESPECT (1993-1995) and RESPECT-2 (1999-2000). RESULTS The participants from RESPECT (n = 2457) and RESPECT-2 (n = 3080) were demographically similar. However, the proportion of participants reporting any unprotected anal sex was much higher in RESPECT-2 (women: 7% vs. 18%; men: 7% vs. 17%). Also, substantially more participants reported a new sex partner in RESPECT-2 (women: 43% vs. 61%; men: 54% vs. 72%). In addition, more women reported 2 or more partners (37% vs. 48%) and a partner with another concurrent sex partner (19% vs. 32%). Slightly more women and men in RESPECT-2 reported 2 protective behaviors, having an HIV test and any condom use; however, consistent condom use did not differ. Conversely, the proportion of participants with bacterial STDs (chlamydia, gonorrhea, or syphilis) was much lower in RESPECT-2 (women: 24% vs. 18%; men: 38% vs. 24%). CONCLUSIONS Despite substantial promotion of safer sex behaviors over the past decade, many risk behaviors were stable over time, and some behaviors, such as unprotected anal sex, appeared substantially higher. Even in the absence of widespread behavior change, the prevalence of common bacterial STDs appeared to have decreased appreciably.