A regional surveillance system for eastern equine encephalitis (EEE) virus was established in central New York in 1984 after the 2nd human EEE fatality occurred in 1983. A six-year interepizootic period was followed by two consecutive years of EEE activity. In 1990 and 1991 seven equine cases each year were accompanied by record numbers of EEE virus isolations from mosquitoes, wild birds and sentinel pheasants. There was a sequence to the appearance of EEE virus at individual swamp complexes and to EEE upland from these swamp complexes. The spatial and temporal distribution of equine cases provide evidence for multiple foci of EEE virus and a role for Cs. melanura in the transfer of EEE virus from swamp to upland areas.