Hantaan-like virus was identified as the cause of three cases of acute renal failure in staff handling laboratory rats on a Belgian university campus. Serological studies revealed infected rat strains at different sites on the campus and evidence of past and recent infections in 39 staff members and of infection in 50% of exposed staff. Hantaan-reactive antibody was very rare in control populations. Staff or rats at three other Belgian institutions had no signs of infection. The origin of the virus was not identified. Infected rats have been removed. These are the first confirmed cases, outside East Asia, of human illness due to rat-borne Hantaan-like virus, and of infection in laboratory rats. Surveillance of rat colonies is also recommended in countries which, like Belgium, have no previously recognised cases of haemorrhagic fever with renal syndrome.