The environmental and social context in which a leptospirosis outbreak took place during the summer of 1996 in the Rio de Janeiro Western Region was examined by using spatial analysis of leptospirosis cases merged with population and environmental data in a Geographical Information System (GIS). Important differences were observed between places where residences of leptospirosis cases are concentrated and other places in the region. Water supply coverage, solid waste collection, sewerage system coverage and flood risk area were the main determining variables from an initial list of ten. The influence of these unfavorable social and environmental factors is verified hundreds of meters distant from the leptospirosis case residences, demonstrating a necessity to broaden the area of health surveillance practices. The geocoding indicated that some cases did not report contact with flood water, even though they were geographically adjacent to cases who did report this contact. Cases may only report exposures they believe are related to the disease. Geocoding is a useful tool for evaluating such bias in the exposure recall.
