A 67-year-old woman presented to our hospital following a several-day history of nonspecific abdominal upset and malaise. Blood work at initial presentation included a white blood cell count (WBC) of 6.3 × 10 9/L, neutrophils 4.8 × 10 9/L with a normal differential, platelets 180 × 10 9/L, hemoglobin 164 g/L, and hematocrit 48%. Acute respiratory failure and shock necessitating mechanical ventilation and hemodynamic support developed 3 days later. Chest computed tomography scan demonstrated diffuse bilateral airspace disease and significant thickening of interstitial septa consistent with pulmonary edema. Heart function was normal on echocardiography. Repeat blood work showed new leukocytosis with WBC 63.1 × 10 9/L, neutrophils 34.6 × 10 9/L, myeloid precursors 4.4 × 10 9/L, and lymphocytes 19.6 × 10 9/L; low platelets of 60 × 10 9/L; and high hemoglobin of 187 g/L with hematocrit 59%.The peripheral blood film included neutrophilia without toxic granulation, myelocytosis, >10% immunoblastic lymphocytes (∼40% counted [inset]), thrombocytopenia, and hemoconcentration. The presence of at least 4 of these 5 morphologic features, in the context of acute noncardiogenic pulmonary edema, is highly sensitive and specific for hantavirus infection. Unfortunately, the patient died within hours of blood film collection. Serum immunoglobulin M and gel polymerase chain reaction results obtained the following week confirmed Sin Nombre hantavirus infection.