Renal immunohistochemistry was studied in 21 patients with Nephropathia epidemica, a benign hemorrhagic fever endemic in northern Europe. Between days 4 and 25 after the onset of fever, scanty but distinct deposits containing immunoglobulins and C3 were detected in glomeruli and tubules. The occurrence of deposits was verified with electron microscopy. It is concluded that an immune complex mechanism plays a part in the pathogenesis of this disease. It is also evident that the febrile period coincides with the presence of immune complexes in the circulation.