An increase in exclusive breastfeeding prevalence can substantially reduce mortality and morbidity among infants. In this paper, estimates of the costs and impacts of three breastfeeding promotion programmes, implemented through maternity services in Brazil, Honduras and Mexico, are used to develop cost-effectiveness measures and these are compared with other health interventions. The results show that breastfeeding promotion can be one of the most cost-effective health interventions for preventing cases of diarrhoea, preventing deaths from diarrhoea, and gaining disability-adjusted life years (DALYs). The benefits are substantial over a broad range of programme types. Programmes starting with the removal of formula and medications during delivery are likely to derive a high level of impact per unit of net incremental cost. Cost-effectiveness is lower (but still attractive relative to other interventions) if hospitals already have rooming-in and no bottle-feeds; and the cost-effectiveness improves as programmes become well-established. At an annual cost of about 30 to 40 US cents per birth, programmes starting with formula feeding in nurseries and maternity wards can reduce diarrhoea cases for approximately $0.65 to $1.10 per case prevented, diarrhoea deaths for $100 to $200 per death averted, and reduce the burden of disease for approximately $2 to $4 per DALY. Maternity services that have already eliminated formula can, by investing from $2 to $3 per birth, prevent diarrhoea cases and deaths for $3.50 to $6.75 per case, and $550 to $800 per death respectively, with DALYs gained at $12 to $19 each.