In 2000, 21 persons were reported with acute illness attributed to West Nile virus (WNV) infection; 19 were hospitalized with encephalitis or meningitis. Of the 21, 10 resided in the Staten Island borough (Richmond County) of New York City. Other ill persons resided in nine other counties--Kings (Brooklyn), New York (Manhattan), and Queens counties in New York; Hudson, Passaic, Monmouth, Morris, and Bergen counties in New Jersey; and Fairfield County in Connecticut. Because ill persons represent only a fraction of the persons who are infected, many more persons probably were infected in 2000. To determine the prevalence of recently acquired WNV infection and associated risk factors for infection, random household cluster serosurveys were conducted in Staten Island and portions of Fairfield County, Connecticut, and Suffolk County, New York, during October-November 2000. All three areas had intense WNV epizootics as determined by avian mortality and mosquito surveillance systems. This report summarizes the preliminary results of this survey and indicates that in areas with intense epizootic WNV activity, asymptomatic or mildly symptomatic human infections can occur.