More than 600 sera from 245 patients with a clinical diagnosis of hemorrhagic fever were preserved by the Hemorrhagic Fever Commission during the Korean Conflict, 1951-1954. These sera were tested for IgM- and IgG-specific antibodies to Hantaan virus by enzyme immunoassay and for hantaviral antigen by immunoassay; one serum from each patient was tested by plaque reduction neutralization using both Hantaan and Seoul viruses. Only 15 patients failed to develop antihantaviral antibodies; most sera contained high titered IgM antibody on admission, and all were IgM-seropositive by day 7 after onset. Attempts to detect hantaviral antigen were unsuccessful. All seropositive patients had highest plaque reduction neutralization titers to Hantaan virus, suggesting that this virus was responsible for the disease seen. These results confirm that hemorrhagic fever of the Korean Conflict was due to Hantaan virus and demonstrate that measurement of specific IgM antibody is the method of choice for diagnosis of acute disease.