This paper investigates the causal effect of Oportunidades, a conditional cash-transfer program in Mexico, on overweight and obesity of adolescents living in poor rural areas. Affecting youth weight was not a goal of this program. However, health economics research suggests that the provision of schooling, health information sessions and sizable cash transfers to Oportunidades participants could have substantially changed their overweight and obesity rates. Exploiting an exogenous jump in program participation by means of a fuzzy Regression Discontinuity (RD) design, the evidence of this paper suggests that Oportunidades decreased obesity among participant women. The identified local average treatment effect (LATE) at the threshold for program eligibility suggests that female obesity would decrease if the program was expanded to cover slightly better-off households. The design of the program does not allow disentangling the causal pathways that contributed to the lower prevalence of obesity among women, but the effect likely resulted from increased access to information and schooling, improved dietary quality, increased monitoring of health outcomes and (possibly) increased physical activity. Suggestive evidence shows that teen pregnancy rates were higher among non-participants. Therefore, weight gain after childbirth might also explain higher obesity rates among non-participant females. Copyright © 2011 John Wiley & Sons, Ltd.