We use data from the National Longitudinal Survey of Adolescent Health to estimate effects of adolescent girls' overweight on their propensity to engage in risky sexual behavior. We estimate single equation, two-stage, and sibling fixed-effects models. We consider both absolute weight and weight relative to other girls in the individual's school. We focus on vaginal intercourse without a condom, any type of sexual activity while under the influence of alcohol, and anal intercourse. Our findings confirm previous research indicating that overweight or obese girls are less likely than their recommended-weight counterparts to be sexually active. As a result, they are less likely to have vaginal intercourse without a condom. However, overweight or obese girls are not less likely to have sex under the influence of alcohol, and once they have had vaginal intercourse, their consistency of condom use is no different from that of their recommended-weight peers. The most striking finding is that overweight or obese girls are at least 15% more likely than their recommended-weight peers to have ever had anal intercourse regardless of whether they have ever had vaginal sexual intercourse. The results from this study underscore the importance of using multifaceted and contemporary measures of risky sexual behavior and have implications for the health and well-being of adolescent girls. (JEL I12, J13)