Background: Numerous studies have examined the association between adolescents' marijuana use and their high-risk sexual behaviors and sexually transmitted diseases (STDs). However, the validity of the findings is questionable because most of the studies relied on self-reporting for measurement of marijuana use and key outcome (i.e., STDs). Goal: The goal was to investigate associations between biologically confirmed marijuana use and laboratory-confirmed STDs and condom use. Study Design: African American females adolescents (n = 522) completed a self-administered survey and face-to-face interview. The adolescents provided urine and vaginal swab specimens that were analyzed for marijuana metabolites and STDs, respectively. Results: Among the study subjects, 5.4% tested positive for marijuana. These adolescents were more likely to test positive for Neisseria gonorrhoeae (adjusted odds ratio [AOR] = 3.4) and Chlamydia trachomatis (AOR = 3.9). They were more likely to have never used condoms in the previous 30 days (AOR = 2.9) and to have not used condoms consistently in the previous 6 months (AOR = 3.6). Conclusion: The findings represent unique biologic evidence that STDs and sexual risk behavior may co-occur with marijuana use. Interventions designed to reduce adolescents' risk of STDs and HIV infection should address marijuana use.