Ribavirin, a broad-spectrum antiviral drug, is active against hemorrhagic fever viruses (with the exception of Ebola virus) in cell culture systems. In model infections with arenaviruses in guinea pigs and monkeys, ribavirin has demonstrated both prophylactic and therapeutic efficacy. In therapeutic studies it has not prevented late-onset neurologic disease. In human cases of Lassa fever, it significantly reduces mortality when administered before day 7 of illness to persons at high risk. In rodents and monkeys infected with Rift Valley fever virus, ribavirin therapy resulted in reduced mortality; prophylactic administration to volunteers infected with sandfly fever virus, Sicilian strain, prevented development of illness. Ribavirin increased the number of survivors and the mean time to death in suckling mice infected with Crimean-Congo hemorrhagic fever virus and in suckling mice infected with Hantaan virus. In the People's Republic of China, ribavirin significantly reduced mortality in patients with hemorrhagic fever with renal syndrome. Ribavirin has not been effective in animal models of filoviral and flaviviral infections. The only important adverse effect of ribavirin in humans is manageable, reversible anemia.