OBJECTIVE: Target 10 of the Millennium Development Goals (MDGs) is to "halve by 2015 the proportion of people without sustainable access to safe drinking water and basic sanitation". Because of its impacts on a range of diseases, it is a health-related MDG target. This study presents cost estimates of attaining MDG target 10. METHODS: We estimate the population to be covered to attain the MDG target using data on household use of improved water and sanitation for 1990 and 2004, and taking into account population growth. We assume this estimate is achieved in equal annual increments from the base year, 2005, until 2014. Costs per capita for investment and recurrent costs are applied. Country data is aggregated to 11 WHO developing country subregions and globally. FINDINGS: Estimated spending required in developing countries on new coverage to meet the MDG target is US42 billion for water and US142 billion for sanitation, a combined annual equivalent of US18 billion. The cost of maintaining existing services totals an additional US322 billion for water supply and US 16 billion for sanitation, a combined annual equivalent of US54 billion. Spending for new coverage is largely rural (64%), while for maintaining existing coverage it is largely urban (73%). Additional programme costs, incurred administratively outside the point of delivery of interventions, of between 10% and 30% are required for effective implementation. CONCLUSION: In assessing financing requirements, estimates of cost should include the operation, maintenance and replacement of existing coverage as well as new services and programme costs. Country-level costing studies are needed to guide sector financing.