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To test the hypothesis that the concentration of angiopoietin-2 relative to angiopoietin-1 may be a useful biological marker of mortality in acute lung injury patients. We also tested the association of concentration of angiopoietin-2 relative to angiopoietin-1 with physiologic and biological markers of activated endothelium.Prospective, observational cohort study.Intensive care units in a tertiary care university hospital and a university-affiliated city hospital.Fifty-six mechanically ventilated patients with acute lung injury.Baseline plasma samples and pulmonary dead-space fraction measurements were collected within 48 hrs of acute lung injury diagnosis.Plasma levels of angiopoietin-1 and angiopoietin-2 and of biomarkers of endothelial activation were measured by enzyme-linked immunosorbent assay. Baseline concentration of angiopoietin-2 relative to angiopoietin-1 was significantly higher in patients who died (median, 58 [interquartile range, 17–117] vs. 14 [interquartile range, 6–35]; p = .01). In a multivariable analysis stratified by dead-space fraction, concentration of angiopoietin-2 relative to angiopoietin-1 was an independent predictor of death, with an adjusted odds ratio of 4.3 (95% confidence interval, 1.3–13.5; p = .01) in those with an elevated pulmonary dead-space fraction (p = .03 for interaction between pulmonary dead-space fraction and concentration of angiopoietin-2 relative to angiopoietin-1). Moderate to weak correlation was found with biological markers of endothelial activation.The ratio of concentration of angiopoietin-2 relative to angiopoietin-1 may be a prognostic biomarker of endothelial activation in acute lung injury patients and, along with pulmonary dead-space fraction, may be useful for risk stratification of acute lung injury patients, particularly in identifying subgroups for future research and therapeutic trials.