Geographic information systems (GIS) are powerful tools for investigating the ecogeography of environmentally acquired infections. GIS technology was used to geocode and map cases of blastomycosis, by household, of human and dog residents of Vilas County, Wisconsin, USA. Human case households (n = 136) were from a comprehensive street address registry 1979--2001; human controls were 200 random-number selected households from 2001 county tax records. Dog cases (n = 116) were from a consecutive street address registry from a private veterinarian practice, Eagle River; dog controls were 200 random-number selected addresses from the 2001 total practice registry. Where geocoding was not available from existing maps, home sites were visited and geocoded using a handheld global positioning system (n = 61). Waterway characteristics were obtained from the Wisconsin Department of Natural Resources GNIS database, soil type data from the USDA. Data were analysed with chi-square and Mann-Whitney tests; cluster analysis with CrimeStat II software. A disproportionate number of human and dog cases were associated with waterways of the western Eagle River area. Human and dog cases were more commonly near waterways < 500 m elevation (more nutrient rich) (P < or =0.001 for both) and were associated with sands (prone to drought) (P < or = 0.01 for both). When the nearest waterway was a lake, case addresses were more commonly near lakes with <7.6 m maximum depth (more lake mixing) (P <0.05, humans and dogs) but mean depth, type and size of lakes did not differ from controls. Further studies are needed to clarify the nature of these associations.