This study is a phase II, randomized, double-blind, placebo-controlled evaluation of intravenous methylprednisolone versus placebo in treatment of hantavirus cardiopulmonary syndrome (HCPS). Patients with suspected or known hantavirus will be randomized to receive intravenous methylprednisolone or placebo over 3 days. Following the completion of this acute phase therapy, patients will be seen for follow up visits on days 14, 28, 84 and 6 months after study entry. Follow up visits will include a physical examination, including vital signs. In addition, blood will be drawn for a blood count, clinical chemistries, and quantitative polymerase chain reaction (day 14). Since Hantavirus pathogenesis involves the pulmonary system, other tests to be performed include chest x ray (day 28) and spirometry (days 28 and 180). The study will require 60 subjects with confirmed Hantavirus infection. Study subjects will include males and females greater than or equal to 2 years of age suspected of having Hantavirus disease. The enrolling co investigator must feel that Hantavirus disease is likely on the basis of the clinical syndrome. The primary study objectives are to: assess the efficacy of intravenous methylprednisolone in reducing the severity of HCPS and assess the safety of methylprednisolone in persons with suspected and proven Hantavirus infection. The secondary objectives are to: assess the impact of therapy on viremia and assess whether measurement of neutralizing antibody titers at entry or Human Leukocyte Antigen (HLA) typing can identify subgroups with increased risk of severe disease and/or death and whether therapy is effective in these subgroups. The primary endpoints will include: the proportion of subjects who develop one or more of the following critical events associated with severe disease 28 days after study entry: death, PaO2/FiO2 ratio less than or equal to 55, cardiac index less than or equal to 2.2, pulseless electrical activity, ventricular tachycardia or fibrillation; and number of serious adverse events determined by study investigators to be at least possibly related to study treatment. For this endpoint researchers will report: the median number of serious adverse events and the proportion that experience one or more serious adverse events. The secondary study endpoints include: to assist in defining the natural history of the disease but will not meaningfully affect treatment: Extracorporeal Membrane Oxygenation (ECMO); duration of intensive care unit stays; duration of hospital stays; duration of shock and/or pressor/inotropic support; length of time on mechanical ventilation; intubated and placed on a ventilator; refractory shock despite fluid resuscitation; and serum creatinine greater than or equal to 3.0 milligrams/deciliter.