Haemorrhagic fever with renal syndrome (HFRS) is a human disease characterized by flu-like symptoms, renal dysfunction, and in severe cases, haemorrhagic manifestations. The causative agents of HFRS are Hantaan (HTN), Seoul (SEO), Puumala (PUU) and Dobrava (DOB) hantaviruses. Hantavirus infections are of increasing importance in Europe. Outbreaks occur in Belgium with a 3- to 4-year interval with an increasing number of cases. We describe the largest outbreak so far in Belgium with 217 serologically and clinically confirmed cases in the period between October 1995 and December 1996. We demonstrated that the use of viral antigen derived from a local PUU-strain was able to detect significantly more sera positive for IgM in an immunofluorescence assay. Furthermore, although in some cases SEO, HTN and DOB antibody-reactivities were detected by ELISA, only PUU infections could be confirmed by neutralization test. The presence of an unknown hantavirus serotype circulating in Belgium should be considered.