Nephropathia epidemica (NE) is a hemorrhagic fever with renal syndrome (HFRS) normally taking a benign clinical course. The etiologic agent, Puumala hantavirus is genetically closely related to Sin Nombre virus, which causes a frequently lethal febrile syndrome with pulmonary involvement (hantavirus pulmonary syndrome, HPS). HPS is characterized by acute respiratory distress, non-cardiogenic pulmonary edema and severe and hypotension, but usually no significant renal involvement. Pulmonary involvement and respiratory symptoms also occur in NE. To understand the mechanisms of pulmonary involvement in NE, we studied the clinical records and chest X-rays of 125 hospital-treated acutely ill NE patients. Twenty-eight percent of the patients had disease-related changes in their chest radiographs. Pleural effusion and atelectasis were the most common X-ray findings, whereas frank pulmonary edema was rare. The patients with pathologic X-ray findings had a more marked hypoproteinemia (lowest measured serum protein concentration 54 +/- 1 g/l) than those with normal X-ray (62.1 +/- 0.9 g/l, p < 0.001) and leukocytosis (highest measured blood leukocyte count 14.1 +/- 0.9 x 10(9)/l vs. 10.6 +/- 0.6 x 10(9)/l, p < 0.001) and more severe renal insufficiency (highest measured serum creatinine 590 +/- 60 mumol/l vs. 356 +/- 29 mumol/l, p < 0.05). Hypoproteinemia best predicted the occurrence of abnormal chest X-ray findings in NE. This suggests, that capillary leakage and inflammation may play a role in the pathogenesis of NE lung involvement, similarly as in HPS. Differently from HPS, the fluid volume overload associated with renal insufficiency seemed to contribute strongly to the chest X-ray changes in NE.