An outbreak of a previously unknown, severe infectious disease with rapidly fatal respiratory distress was reported from Southwestern USA in May 1993. The case fatality rate was about 60 per cent. Using PCR hantavirus RNA could be demonstrated in lungs and other tissues from patients. Mice and others rodents, e g Peromyscus maniculatus, were shown to be a reservoir of the virus. This new virus, designated Four Corner virus, has been further characterised and seems to be closely related to Puumala virus that causes nephropathia epidemica (hemorrhagic fever with renal syndrome) in Scandinavia and Central Europe.