Bronchiolitis obliterans syndrome in lung transplant recipients. The leading cause of late graft loss after lung transplantation is bronchiolitis obliterans syndrome. The process is a manifestation of chronic rejection, and is characterized by an excessive fibroproliferation in the small airways, leading progressively to luminal obliteration and graft injury. Both alloantigen-dependent (acute rejection, histocompatibility) and alloantigen-independent (ischaemia-reperfusion injury, cytomegalovirus infection, gastroesophageal reflux disease) risk factors may contribute to the development of the disease. Early in the process, damage to the airway epithelium occurs, which then triggers a massive influx of alloreactive T-cells into the graft tissue. Activated T-cells release a wide range of cytokines and growth factors, which in turn are capable of stimulating cellular proliferation and matrix protein synthesis in fibroblasts as well as in airway smooth muscle cells. Clinically, a decline in lung functions together with nonspecific symptoms can usually be observed in these patients, while later in the disease course recurrent respiratory tract infections are more common. Up till now, no effective therapy is available for bronchiolitis obliterans syndrome, however, certain immunosuppressive regimens may slow down the progression of the disease.