INTRODUCTION: We have previously documented the relationships between stress and depression symptoms and adolescent women's nonuse and misuse of condoms and other contraceptive methods and on their unintended pregnancy rates. AIM: Here, we examine relationships between mental health symptoms and another understudied adolescent reproductive health behavior-frequency of sexual intercourse. MAIN OUTCOME MEASURE: Our outcome was weekly sexual intercourse activity. METHODS: We used panel data from a longitudinal, population-based cohort study of 992 women ages 18-20. Weekly journals measured sociodemographic, relationship, reproductive, and mental health characteristics, sexual and contraceptive behaviors, and pregnancy history. We examined 27,130 surveys from 952 women during the first study year. Predictors of weekly sexual intercourse were moderate to severe stress (Perceived Stress Scale-4) and depression (Center for Epidemiologic Studies Depression Scale-5) symptoms measured at baseline. Multilevel, mixed-effects logistic regression models estimated the relationships between stress and depression symptoms and the weekly odds of sexual intercourse while adjusting for covariate fixed effects and random woman effects. RESULTS: Nearly a quarter of the sample had moderate to severe stress (23%) and depression (24%) symptoms at baseline. Women reported sexual intercourse in 36% of weeks. Proportions of sexually active weeks were higher among women with stress (43%) and depression (40%) compared with those without symptoms (35% and 35%, respectively; P values<0.001). Controlling for covariates, women with baseline stress symptoms had 1.6 times higher weekly odds of sexual intercourse compared with women without stress (adjusted odds ratio 1.6, confidence interval [1.1, 2.5]; P=0.04). Depression symptoms were not associated with sexual intercourse frequency in adjusted models. CONCLUSIONS: Stress symptoms were positively associated with sexual intercourse frequency among these young women. Research and practice efforts are needed to identify effective sexual health promotion and risk-reduction strategies, including contraceptive education and counseling, in the context of mental health symptoms and unintended pregnancy.