The aim of our study was to analyze the clinical course and outcome of acute renal failure (ARF) in patients with hemorrhagic fever with renal syndrome (HFRS). From 1983 to 1995, we treated 33 patients (27 males, 6 females) aged from 16 to 71 years. Half of patients were connected with work at a farm or in a forest. The disease was confirmed serologically with indirect immunofluorescence test (IFT) and enzyme-linked immunosorbent assay (ELISA). In 18 patients percutaneous kidney needle biopsies were analyzed. In 85% of the cases, the disease broke out from June to October. The most frequently expressed clinical signs and symptoms were fever, nausea/vomiting, headache, backache, abdominal pain, myalgia, diarrhea, conjunctival injection, and hemorrhages. Four patients had concomitant pancreatitis. In 25 patients, oliguria was present, and transient hemodialysis treatment was needed in 19 patients. Infection with Hantaan virus was established in 20 patients and with Puumala virus in 13 patients. At renal biopsy, acute interstitial nephritis accompanied with hemorrhages and necrosis was found, and at a later biopsy there were also signs of interstitial fibrosis. All patients were cured, but renal function was not completely recovered in some. We conclude that ARF is a serious complication in patients with HFRS. Although not lethal in our group of patients, many of them showed severe signs and symptoms of illness. Transient hemodialysis was necessary in two-thirds of the patients. Some degree of functional defects and morphological changes might persist.