Three hundred million individuals are at risk of infection by schistosomes and around 200,000 die each year of this disease. Severe clinical disease in schistosomiasis is often the consequence of heavy infection which, in several endemic areas, are determined largely by the susceptibility/resistance of individuals. Previously, we reported evidence, based on a segregation analysis in Brazilian pedigrees, that intensity of infection by Schistosoma mansoni was influenced by a major gene, indicating that host genetic factors are probably critical in controlling schistosome infection and disease development. To localize this gene, referred to as SM1, we performed a genome-wide study on 142 Brazilian subjects belonging to 11 informative families Our results show a linkage to only one region, on chromosome 5q31-q33, with maximum two-point lod scores of +4.74 and +4.52 for D5S636 and the colony stimulating factor-1 receptor marker (CSF1R), respectively. This was corroborated by multipoint analysis, indicating a close proximity to CSF1R as the most likely location of SM1. This region contains several candidate genes encoding immunological molecules that were shown to play important roles in human protection against schistosomes.