Diagnosis and prognosis of acute respiratory distress syndrome (ARDS) depend on clinical criteria that are confounded by ARDS heterogeneity. Clinical diagnostics miss 40% of ARDS diagnoses and only 34% of ARDS are recognized at the time of ARDS diagnostic criteria fulfillment. Biomarker-based approaches may improve diagnostic/prognostic strategies and tailor patient-specific therapies, but no validated biomarkers exist. Matrix metalloproteinase-3 (MMP3) is a proteolytic enzyme involved in the pathophysiology of acute respiratory distress syndrome (ARDS) that may serve as a biomarker in ARDS. This study characterized MMP3 in plasma from patients enrolled in the Albuterol for the Treatment of Acute Lung Injury (ALTA) trial to determine the prognostic value of MMP-3 in ARDS. MMP-3 was measured in plasma samples by enzyme-linked immunosorbent assay. MMP-3 discriminated between healthy controls and ARDS. MMP-3 levels were elevated on day three among non-survivors, and increased levels from enrollment to day three predicted mortality.