In areas of seasonal malaria transmission the burden of severe disease and mortality due to malaria is mainly among children under 5 years of age. Intermittent preventive treatment (IPT) with antimalarial drugs given to all children once a month during the transmission season is a promising new strategy for malaria prevention. Studies in Senegal, Ghana, Mali and The Gambia have shown this approach can be highly effective. In Senegal, seasonal IPT with sulfadoxine-pyrimethamine (SP) and one dose of artesunate resulted in a 90% reduction in incidence of clinical malaria in a recent trial in Senegal (Cisse et al., Lancet 2006). The purpose of the present project is to determine the public health impact and cost effectiveness of this intervention when it is delivered through the routine health service to communities in rural areas in Senegal. Demographic surveillance will be set up in the rural population of three districts (Mbour, Bambey and Fatick) which comprises approximately 540,000 people, including 100,000 children under 5 yrs, and is served by 54 health posts, as an expansion of the area covered by the existing DSS of Niakhar. Information about births, deaths and migrations, household characteristics such as socioeconomic status, and vaccination status of children and their use of bednets, will be recorded in 6-monthly rounds of all households. In selected areas, deaths among children under 10 years will be investigated using verbal autopsies. Over four years from September 2008 - November 2011, seasonal IPT (three monthly administrations of SP (sulfalene-pyrimethamine) plus amodiaquine during the transmission season each year to children 3-59 months of age) will be introduced gradually, in a step-wedge design, by 9 health posts in 2008, by an additional 18 posts in 2009, and another 18 in 2010 and 9 in 2011. At the end of each transmission season, a cross-sectional survey of 2400 children under 5 yrs of age, in which finger prick blood samples will be taken, will be used to estimate the prevalence of molecular markers of drug resistance to Plasmodium falciparum, the prevalence of anaemia and the nutritional status of children. Malaria incidence will be monitored by passive surveillance through health posts, health centres, and hospitals. Cost effectiveness will be assessed.