This study uses a randomized controlled trial (RCT) to evaluate the effectiveness of a mass media campaign designed to reduce pregnancy among Latina teenagers. We aired public service announcements (PSAs) on terrestrial radio stations in California. The sample includes 28 radio stations and over 11,000 births to Latina teenagers in 2016. The PSA, narrated by a young Latina, stressed the opportunity costs to teenage mothers of having a baby, e.g., missed opportunities for fun, socializing with friends, or attending school. Although this message expresses themes articulated by Latina teens and young mothers whom we interviewed, geocoded birth records suggest the PSAs had little effect. With data on all births in the state obtained from the California Department of Public Health, we can reject the hypothesis that the PSAs reduced births to Latina teenagers by 9% or more based on the 90% confidence interval of the estimated average treatment effect. In addition to shedding light on this particular radio campaign, this study provides a template for the design and analysis of mass media campaigns to improve other public health outcomes.