In this study we demonstrate that standard histological evaluation of lung biopsies significantly underestimates eosinophil accumulation when compared to assessments using a novel eosinophil peroxidase (EPX)specific monoclonal antibody (EPX-mAb) visualized by immunohistochemistry. We employed this unique increase in sensitivity to detect eosinophils and evidence of tissue degranulation in a group of ALI patients. A retrospective assessment of ALI patients who had a lung biopsy taken during the course of their acute care was conducted, comparing the results of these assessments with lung tissue from control subjects. These blinded assessments revealed that the added sensitivity of EPX-mAb immunohistochemistry detected a significant increase in eosinophil accumulation/degranulation in ALI vs. control subjects. More importantly, EPX-mAb immunohistochemistry identified a subset of ALI patients who survived hospitalization, suggesting its use as a prognostic indicator may represent a previously underappreciated diagnostic strategy in the management of pulmonary patients.