INCLUSION CRITERIA:

1. Suspected or proven infection
2. Hypoxemia: PaO2/FiO2is ≤300 mm Hg
3. Bilateral infiltrates consistent with pulmonary edema
4. Positive-pressure mechanical ventilation through an endotracheal tube
5. No clinical evidence of left atrial hypertension to explain bilateral infiltrates
6. Presence of at least three of the four SIRS criteria. If only two criteria are evidenced, one must be temperature or WBC

Criteria 2 and 3 must occur within a 24-hour interval. The 48-hour enrollment time window begins when criteria 2, 3, and 4 are met.

EXCLUSION CRITERIA:

1. <18 years
2. Inability to obtain consent
3. Patient, surrogate, or physician not committed to full support
4. Moribund state in which death was perceived to be imminent
5. Morbid obesity
6. Malignancy or other irreversible disease or condition for which 6-month mortality is estimated to be >50%
7. Known HIV positive with known end stage processes
8. Prior cardiac arrest requiring CPR without fully demonstrated neurological recovery; or New York Heart Association Class IV
9. Pregnant or nursing
10. ALI/ARDS induced by mechanical or chemical injury directly to the lung (including burns, trauma, and near drowning)
11. >48 hours since all inclusion criteria are met
12. Neuromuscular disease that impairs ability to ventilate without assistance
13. Severe chronic respiratory disease, severe pulmonary hypertension, or ventilator dependency
14. Chest wall deformity resulting in severe exercise restriction, secondary polycythemia, or respirator dependent
15. History of organ transplant (including bone marrow)
16. Severe chronic liver disease, as determined by a Child-Pugh Score >10
17. Hemoglobin persistently < 7.0 g/dL
18. Platelet count <50,000/mm3
19. Prolonged INR >3
20. Bleeding disorders unless corrective surgery has been performed
21. Active internal bleeding
22. Major surgery within 24 hours before study drug infusion, or evidence of active bleeding postoperatively, or plan for any major surgery within 3 days after study drug infusion.
23. Diffuse alveolar hemorrhage from vasculitis
24. Known bleeding diathesis
25. Presence of an epidural catheter or lumbar puncture within 48 hours before study drug infusion or anticipation of receiving an epidural catheter or a lumbar puncture within 48 hours after study drug infusion
26. Stroke within 3 months of study entry
27. Trauma with an increased risk of life-threatening bleeding
28. A history of severe head trauma that required hospitalization, or intracranial surgery within two months of study entry
29. Any history of intracerebral arteriovenous malformation, cerebral aneurysm, or central nervous system mass lesion
30. Uses of certain medications or treatment regimens such as chemotherapy, unfractionated heparin, low-molecular-weight heparin, Warfarin, antithrombin III, acetylsalicylic acid, glycoprotein IIb/IIIa antagonists, thrombolytic therapy, and activated Protein C are restricted.
31. Participation in another experimental medication study within 30 days of study entry.