The Economic Commission for Latin America and the Caribbean (ECLAC) jointly with the World Program of Food (WFP) and recognized experts of the region developed a methodology that, using secondary information, estimate the opportunity cost derived from undernutrition. This methodology has been successfully applied in Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, Panama and the Dominican Republic, where the cost of undernutrition was estimated at 6.7 billion dollars in 2004. The present study covers four countries in South America: Bolivia, Ecuador, Paraguay and Peru. The results indicate that the cost of the malnutrition in these countries reached 4.3 billion dollars in 2005, which is equivalent to 3.3 per cent of the GDP of these countries. The results strongly point out that child undernutrition is not only a problem of health or an unacceptable situation ethically, but it is a national problem, given the enormous social costs and the loss of opportunities that it imposes on the national economy.