Dengue and Aedes aegypti are widespread in the State of Maranhão. During 1995, 87 of the 136 of the State's counties, including 176 towns and 480,687 households, were studied, of which 30 counties (34.4%), 118 towns (67.0%) and 10,357 households (2.1%) were positive for Aedes aegypti. The positive counties are distributed as follow: 3 on São Luís Island, 7 in the Amazonia of Maranhão, 12 in the southern zone of the cerrados or savannas, and 5 in the mixed forest/savanna/palm grove zone. One positive county for Ae. aegypti was located in each of the following: alluvial fields, forest/palm grove, and sand dune/shoals areas. The Ae. aegypti-positive household rates were higher in the Amazonia of Maranhão (3.5%) and on São Luís Island (2.5%), because they are the most heavily traveled migratory and trade routes between Maranhão and neighboring states and are also the State's main economic centers. Household infestation rates for Ae. aegypti and reported dengue case rates were higher during the rainy season, showing the importance of rain in forming prime breeding sites for Aedes and spreading dengue.