The prevalence of antibodies against glomerular basement membrane (GBM) antigens in sera from 47 patients with serologically verified hantavirus (Puumala serotype) infection were investigated. Antibodies were measured by immunoassays using as antigen a crude human GBM preparation and the NC1 portion of type IV collagen (Goodpasture antigen), respectively. Seventy-seven percent of the patients had IgM antibodies against the non-Goodpasture glomerular basement membrane (non-GP GBM) in their acute phase serum samples as compared to only 4% seropositivity rate in convalescent phase samples. However, there was no correlation between the non-GP GBM IgM antibody levels and the decrease in GFR as measured by Cr EDTA clearance. None of the patients had antibodies against the Goodpasture antigen. In a control group of 10 patients with influenza and parainfluenza infections serum IgM against non-GP GBM could not be demonstrated, neither in acute phase nor in convalescent phase sera. The possible pathophysiological implications of these findings are discussed.