The Puumala virus is the causative agent of nephropathia epidemica, a European form of hemorrhagic fever with a renal syndrome. From its reservoir in bank voles, the virus is spread by airborne transmission to humans. Occupational risks for the acquisition of nephropathia epidemica are not well defined. The prevalence of serum antibodies to Puumala virus was determined for Swedish farmers. From a comparison of the prevalence among farmers from various parts of the country, the assumption that Puumala virus occurs endemically only in the northern and central parts of Sweden was also tested.Serum samples from 910 farmers and 663 referents living in various rural parts of Sweden were tested with an enzyme-linked immunosorbent assay, using a recombinant nucleocapsid protein of Puumala virus as the antigen.North of a latitude of 59 degrees N, the prevalence of Puumala virus antibodies was significantly higher among farmers (12.9%) than among referents (6.8%). In the southern areas, antibodies to Puumala virus were rare, and altogether only 2 of 459 persons had antibodies. Seropositive persons did not differ from seronegative ones with regard to blood pressure, and they did not comprise cases of chronic renal disease.Serological evidence confirmed that the exposure of humans to Puumala virus is firmly restricted to the northern and central parts of Sweden. In addition the evidence indicated that, in this region, farming is associated with an increased risk of contracting hantavirus infection.