Fil: Harrison, Lee H. University of Pittsburgh Graduate School of Public Health,Pittsburgh, Pennsylvania; Estados Unidos.Fil: Halsey, Neal A. Department of International Health, Johns Hopkins Medical Institutions, Baltimore, Maryland; Estados Unidos.Fil: McKee, Kelly T. Womack Army Medical Center, Fort Bragg, North Carolina; Estados Unidos.Fil: Peters, Clarence J. Special Pathogens Branch, Centers for Disease Control, Atlanta, Georgia; Estados UnidosFil: Barrera Oro, Julio G. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Virales Humanas; Argentina.Fil: Briggiler, Ana M. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Virales Humanas; Argentina.Fil: Feuillade, Maria Rosa. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Virales Humanas; Argentina.Fil: Maiztegui, Julio I. ANLIS Dr.C.G.Malbrán. Instituto Nacional de Enfermedades Virales Humanas; Argentina.Argentine hemorrhagic fever (AHF) is a potentially lethal infection in Argentina. The case-fatality ratio is >15%, but treatment reduces the mortality rate to <1%. Diagnosis is based on clinical and laboratory criteria, but no case definition has been validated. A chart review was conducted for patients hospitalized with suspected AHF. Individuals with a fourfold rise in antibody titer were classified as cases. The combination of a platelet count of <100,000/mm3 and a white blood cell (WBC) count of <2,500/mm3 had a sensitivity and specificity of 87% and 88%, respectively, thus suggesting that the use of these criteria in a case definition would be helpful for epidemiological studies of AHF. The combination of a platelet count of <100,000/mm3 and a WBC count of <4,000/mm3 had a sensitivity of 100% and a specificity of 71%; the use of these criteria in a case definition should be helpful for screening patients for therapy with immune plasma in the region where AHF is endemic.