Haemorrhagic fever with renal syndrome (HFRS) is caused by a group of RNA viruses within the family of Bunyaviridae known as hantaviruses. The classical, severe form of HFRS is characterized by fever, headache, abdominal and lumbar pain, proteinuria, haemorrhagic phenomena, shock and renal failure. The disease is associated with the prototype Hantaan virus and occurs in rural areas of Korea and China with Apodemus mice as reservoir hosts. A clinically less severe form of HFRS, which is caused by Seoul virus, occurs in urban areas with the house rat Rattus novegicus as the main reservoir host. The disease in nonendemic areas may be atypical and patients with symptoms the hepatitis and minimal renal involvement have been observed in Malaysia. Outbreaks of HFRS in humans involving infected laboratory rat colonies have occurred in several medical centres in various countries. Hantaviruses cause a chronic, asymptomatic infection in rodents which excrete the virus in their lungs, saliva and urine. Man becomes infected mainly by inhalation of infected droplets from healthy rodent carriers. Seroepidemiological studies using mainly the indirect immunoflourescent antibody test of sera from humans and rats showed that hantaviruses have a worldwide distribution.