In the summer of 1999, West Nile virus was recognised in the western hemisphere for the first time when it caused an epidemic of encephalitis and meningitis in the metropolitan area of New York City, NY, USA. Intensive hospital-based surveillance identified 59 cases, including seven deaths in the region. We did a household-based seroepidemiological survey to assess more clearly the public-health impact of the epidemic, its range of illness, and risk factors associated with infection.