Puumala virus belongs to the hantavirus group and as other members of the group it can cause acute renal insufficiency. Other hantaviruses are responsible for Korean hemorrhagic fever and other hemorrhagic fevers with renal involvement. In Scandinavia, hantavirus nephropathy is better known as nephropathia epidemica. Hantavirus cause asymptomatic infections in mice and rats and the virus is thought to be transmitted to man via inhalation of desicated saliva, urine or feces from infected animals. In the other Nordic countries, the bank vole (Clethrionomys glareolus) is the commonest natural host. Laboratory infections from rats have been reported from all over the world. The disease presents with fever, back and/or abdominal pain, acute renal insufficiency and a bleeding tendency. The latter is usual less pronounced in the European than in the Asian forms of hantavirus infection. The disease is commoner in men and is endemic in forested areas. The greatest incidence is seen in those years when the numbers of mice are greatest. The mortality in nephropathia epidemica appears to be less than 0.5% in contrast to a mortality of about 10% in Korean hemorrhagic fever. Diagnosis is made by demonstration of a significant increase in antibody titer to hantavirus. Treatment is symptomatic and consists primarily of restitution of fluid and electrolyte balance, at times with dialysis. After recovery from hantavirus infection renal function returns to normal. The incidence and prevalence of nephropathia epidemica in Denmark are unknown. The first two cases of Puumala virus infection in Denmark are reported.