The Seoul virus is an important etiologic agent in hemorrhagic fever with renal syndrome (HFRS), and infections with the Seoul virus are less severe than those with the Hantaan virus. However, the information on HFRS caused by the Seoul virus is limited in Korea. Retrospective clinical analysis was done on 30 patients with Seoul virus infection who had been diagnosed as having HFRS by clinical features and serologic testing by the plaque reduction neutralization test from 1986 to 1991 at the Seoul National University Hospital. They were compared with 69 patients with Hantaan virus infection. The Seoul virus was the etiologic agent in 25% of Korean HFRS and the major cause of HFRS during the summer season although infections occurred throughout the year. The Seoul virus infection had a milder degree of bleeding and renal derangement but had severer liver dysfunction than the Hantaan virus infection. Renal histopathologic findings revealed a milder degree of hemorrhage and vascular changes than cases involving Hantaan virus infection. The precise mechanisms of vascular dysfunction and organ involvement in Seoul virus infection, however, still remain to be explored.