To explore the endogenous interferon levels in patients of Argentine hemorrhagic fever (AHF) with different clinical evolution of the disease, 29 fatal and 33 surviving cases of AHF were analyzed. As previously reported, the titers of endogenous alpha-IFN in patients with AHF are very high, generally between 2,000 and 64,000 IU/ml. Thus far, these are the highest levels of circulating interferon detected in any human viral disease. In this study it was found that during the second week of evolution the titers of interferon were significantly higher in fatal cases than in survivors. Therefore, very high levels of interferon have a prognostic value in AHF.