Plasma levels of cytokines were measured by EIA in 15 subjects hospitalized with nephropathia epidemica, a European form of hantavirus-induced hemorrhagic fever with renal syndrome. Concentrations of tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 were increased in all patients at admission, and the concentration of IL-10 was increased in most. TNF-alpha concentrations were still increased 1 week after onset of disease; levels of IL-6 and IL-10 were normalized. TNF-alpha was undetectable by the WEHI cell assay in serum samples obtained throughout the acute phase of disease. Serum levels of the two soluble TNF receptors p55 and p75 correlated with levels of the cytokine, indicating that receptor binding may be the reason for lack of bioactivity in vitro. TNF-alpha is known to induce pathophysiologic and clinical changes similar to those seen in nephropathia epidemica and in diseases caused by other hantaviruses.