Objective:To determine whether idiopathic pneumonia syndrome (IPS), a form of noninfectious lung injury that follows bone marrow transplantation, is associated with cytokine activation and increased susceptibility to lipopolysaccharide (LPS).Design:Case series.Setting:Tertiary referral center for marrow transplantation.Patients:Recipients with biopsy-confirmed IPS; normal volunteers and marrow transplant recipients without IPS were analyzed as controls.Measurements and Main Results:Levels of lymphocyte and macrophage-derived cytokines as well as components of the LPS, LPS-binding protein (LBP), and CD14 system in bronchoalveolar lavage (BAL) fluid were determined. We found evidence of increased vascular permeability (BAL protein) and inflammatory cytokine activation (interleukin-1, interleukin-2, interleukin-6, and tumor necrosis factor-α) in patients with IPS. Patients without IPS had BAL fluid cytokine and protein levels that were similar to levels in BAL fluid from normal volunteers. Moreover, components of the LPS amplification system (LBP and soluble CD14) were increased in patients with IPS but not in patients without IPS.Conclusions:These results provide direct evidence for proinflammatory cytokine activation in IPS and suggest that these patients might be at increased risk for LPS-mediated injury through the LBP amplification pathway.