Purpose-Teenage pregnancy and marijuana use are associated with higher risk of contracting Sexually Transmitted Infections (STIs). In this study, we examined the role of early and current marijuana use as it related to STI risk in a sample of young women who were pregnant teenagers, using a variety of statistical models.Methods-279 pregnant adolescents, ages 12-18, were recruited at an urban prenatal clinic as part of a study that was developed to evaluate the long-term effects of prenatal substance exposure. Six years later, they were asked about their substance use and sexual history. The association of early and late marijuana use to lifetime sexual partners and STIs was examined, and then structural equation modeling (SEM) was used to illustrate the associations among marijuana use, number of sexual partners, and STIs.Results-Bivariate analyses revealed a dose-response effect of early and current marijuana use on STIs in young adulthood. Early and current marijuana use also predicted a higher number of lifetime sexual partners. However, the effect of early marijuana use on STIs was mediated by lifetime number of sexual partners in the SEM, whereas African-American race, more externalizing problems, and a greater number of sexual partners were directly related to more STIs.Conclusions-Adolescent pregnancy, early marijuana use, mental health problems, and African-American race were significant risk factors for STIs in young adult women who had become mothers during adolescence. Pregnant teenage girls should be screened for early drug use and mental health problems, because they may benefit the most from the implementation of STI screening and skill-based prevention programs. Please address all correspondence to: Natacha De Genna, PhD, Program in Epidemiology, Western Psychiatric Institute and Clinic (WPIC), Webster Hall, Suite 138, 4415 Fifth Avenue, Pittsburgh, PA, 15222, degennan@pitt.edu, Telephone: (412) Fax: (412) 246-6875. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Compared to other developed countries, American 15-19 year olds have one of the top three highest reported rates of syphilis, gonorrhea, and chlamydia (Panchaud, Singh, Feivelson, & Darroch, 2000). Within the US adolescent population, girls (especially African-American girls) are at higher risk of contracting a sexually-transmitted infection (STI) than boys (Aral & Holmes, 1990;Bunnell et al., 1999; CDC, 2005;Weinstock, Berman, & Cates, 2000). Although all sexually-active girls are vulnerable to STIs, those who become teenage mothers are a particularly high-risk subset within this age ...