The spatial and temporal distributions of dengue cases reported during a 1991-1992 outbreak in Florida, Puerto Rico (population = 8,689), were studied by using a Geographic Information System. A total of 377 dengue cases were identified from a laboratory-based dengue surveillance system and georeferenced by their residential addresses on digital zoning and U.S. Geological Survey topographic maps. Weekly case maps were generated for the period between June and December 1991, when 94.2% of the dengue cases were reported. The temporal evolution of the epidemic was rapid, affecting a wide geographic area within seven weeks of the first reported cases of the season. Dengue cases were reported in 217 houses; of these 56 (25.8%) had between two and six reported cases. K-function analysis was used to characterize the spatial clustering patterns for all reported dengue cases (laboratory-positive and indeterminate) and laboratory-positive cases alone, while the Barton and David and Knox tests were used to characterize spatio-temporal attributes of dengue cases reported during the 1991-1992 outbreak. For both sets of data significant case clustering was identified within individual households over short periods of time (three days or less), but in general, the cases had spatial pattern characteristics much like the population pattern as a whole. The rapid temporal and spatial progress of the disease within the community suggests that control measures should be applied to the entire municipality, rather than to the areas immediately surrounding houses of reported cases. The potential for incorporating Geographic Information System technologies into a dengue surveillance system and the limitations of using surveillance data for spatial studies are discussed. Through use of the Geographic Information System (GIS), the spatial and temporal distributions of dengue cases reported during a 1990-91 outbreak in Florida, Puerto Rico, were reviewed. The GIS, a computer system that can store, assemble, manipulate, and analyze geographically referenced material, offers a new approach to the study of disease patterns. A total of 377 dengue cases were identified from a laboratory-based dengue surveillance system and georeferenced by their residential addresses on digital zoning and US Geological Survey topographic maps. Weekly case maps were generated for the period June-December 1991, when 94.2% of dengue cases were reported. The epidemic's temporal evolution was rapid, affecting a wide geographic region within 7 weeks of the first reported cases of the season. Cases were reported in 217 houses, 56 (25.8%) of which had 2-6 cases each. Both K-function analysis, and the Barton and David-Knox tests, revealed significant case clustering within individual households over a period of 3 days or less. In general, however, cases had spatial pattern characteristics similar to the population pattern as a whole. The rapid spatial and temporal progress of dengue cases within the community suggests that vector control measures (e.g., source reduction) should be applied to the entire municipality, rather than to the areas immediately surrounding houses of reported cases.