In 1958, Junin virus emerged into the scientific consciousness as the causative agent of Argentine hemorrhagic fever (AHF). 55 Three other members of the arenavirus family have subsequently been implicated in South American hemorrhagic fevers (SAHF): Machupo virus, first isolated in 1963, 34 causes Bolivian hemorrhagic fever (BHF); Guanarito virus, first isolated in 1990, 63 causes Venezuelan hemorrhagic fever (VHF); and Sabiá virus, first isolated in 1990, 15 is a causative agent of hemorrhagic fever in Brazil. All SAHF viruses are associated with a primary rodent reservoir and are transmitted to humans primarily via inhalation of aerosolized virus found in rodent excreta. In 1993, a previously unknown hantavirus, later referred to as Sin Nombre virus (SNV), was identified as the causal agent of a severe, life-threatening respiratory disease now known as hantavirus pulmonary syndrome (HPS), which first appeared in dramatic fashion during an outbreak in the Southwestern United States. 9,52 Subsequent work has uncovered at least eight additional distinct hantaviruses throughout the Western hemisphere associated with HPS. 64 All hantaviruses are associated with a primary rodent reservoir, and their transmission to humans is believed to involve mechanisms similar to human infection with South American hemorrhagic fever viruses. This article describes laboratory, epidemiologic, and clinical features of the SAHF and hantavirus disease in the Americas, and points out common features of their emergence. Because of their vastly different ecology and epidemiology as mosquito-borne diseases, yellow fever and dengue hemorrhagic fever, which also exist as viral hemorrhagic diseases in the Western Hemisphere, are not discussed.