Rates of exclusive breastfeeding are slowly increasing, but remain sub-optimal globally despite the health and economic benefits. This study estimates the costs of not breastfeeding across seven countries in Southeast Asia and presents a cost-benefit analysis of a modeled comprehensive breastfeeding strategy in Viet Nam, based on a large program. Modelling of cost-benefit for Viet Nam used program data on costs combined with effects from a large-scale cluster randomized breastfeeding promotion intervention with controls. This study found that over 12,400 preventable child and maternal deaths per year in the seven countries could be attributed to inadequate breastfeeding. The economic benefits associated with potential improvements in cognition alone, through higher IQ and earnings, total $1.6 billion annually.