Hantavirus pulmonary syndrome is an acute pneumonitis with a high mortality rate that is caused by a newly recognized hantavirus. Four Corners virus (also known as Muerto Canyon virus and Sin Nombre virus) is enzootic among deer mice (Peromyscus maniculatus). Incidental transmission to humans can result in a disease characterized by rapidly progressive respiratory insufficiency, diffuse noncardiogenic pulmonary edema, vascular volume contraction with hemoconcentration, lactic acidosis, depressed cardiac output, and cardiac dysrhythmias preterminally. The onset of pulmonary edema is preceded by a prodrome of fever and severe myalgias. Characteristic laboratory abnormalities include thrombocytopenia, leukocytosis with prevalent immature myeloid cells, and the presence of immunoblastic lymphocytes in the peripheral blood. Agent-specific diagnosis is based on the detection of Four Corners virus-specific antibodies in serum by Western blot assays and the detection of Four Corners virus RNA in peripheral blood mononuclear cells by the reverse transcriptase-polymerase chain reaction. Effective treatment depends on the rapid institution of intensive care support.