In Mexico, the potential impact on child malnutrition from a nutritional supplement (papilla) delivered through a conditional transfer program (Oportunidades) was attenuated by problems of household utilization. A behavioral change through communication intervention was developed to improve supplement utilization. Our study assessed the efficacy of this intervention through the results of a randomized trial. In 2 states (Veracruz and Chiapas) 2 clusters of communities were randomly assigned to intervention or control groups. Data were obtained from 176-198 mothers in intervention and control communities using a survey questionnaire at preintervention baseline and at a 5-mo follow-up. Concordance between reported and observed behaviors was examined through an observational substudy. The 4 behavioral recommendations were: 1) prepare papilla as a pap; 2) administer the preparation every day; 3) administer it between breakfast and dinner; and 4) administer it only to target children. The intervention resulted in a significant increase (P < 0.05) in the prevalence of reported correct behaviors in the intervention group compared with the control for 3 of the behaviors: a mean increase of 42.5% for preparing papilla as pap, 64.4% for daily administration, and 61.5% for giving papilla between breakfast and dinner. Administering to a target child increased significantly in Veracruz (from 51.5% to 90.6%), but not in Chiapas (20.6% to 33.3%). Reported behaviors agreed with observed behaviors in the substudy. With the exception of the target-child administration in Chiapas, adopting the recommendations was culturally acceptable and feasible. The results indicate that improvements in household utilization of the supplement can be achieved with a communication intervention that is potentially feasible for implementation on a large scale within the Oportunidades Program. J. Nutr. 137: 440-446, 2007.