Cancer registries are increasingly applying geographic information system (GIS) technology to their cancer data.4 According to the World Health Organization, a GIS is “a computer-aided database management and map technology that organizes and stores large amounts of multi-purpose information” and “adds the dimension of geographic analysis to information technology (IT) by providing an interface between the data and a map.”5 Registries can use GIS technology to geocode their cancer data, which entails assigning geographic identifiers (geocodes) to computer records of cancer cases and cancer deaths, thereby tying the information in these records to a geographic location or space.6 Geocoding permits the creation of visual representations of geographic patterns of cancer incidence and mortality rates in maps.6 By linking geocoded cancer data sets to other geocoded data sets, including demographic, socioeconomic, environmental, and health services data sets, it also becomes possible to identify and analyze relationships between cancer cases and deaths and a number of other variables.6