Bronchiolitis obliterans (BO) is the pathologic manifestation of chronic allograft rejection in lung transplant recipients and specific diagnosis requires invasive tests. BO causes progressive obstruction of the small airways. The term bronchiolitis obliterans syndrome (BOS) is a clinical surrogate for the histopathologic diagnosis of BO and is measured by lung function testing. KL-6 is a glycoprotein expressed on pulmonary epithelial cells and it is present in the serum of normal individuals in small amounts. Serum KL-6 has been shown to be a useful marker of disease activity in interstitial lung diseases. We demonstrated that serum levels of KL-6 are elevated in lung transplant recipients with BOS when compared with those without BOS and healthy controls. Our results indicate that serum KL-6 measurement has the potential to serve as a noninvasive diagnostic test for the detection of BO in lung transplant recipients.