Endotheliitis is a major sign of graft rejection. Recipient-derived endothelial cells found in two series of liver and kidney transplants were related to graft rejection. Here, we assessed the presence and the number of chimeric endothelial cells in lung transplants, and their relation with graft rejection. In six males grafted with female lungs out of 193 lung transplantations, endothelial chimerism was studied by combined XY-fluorescent in situ hybridization with CD31 and CD45 immunostainings and blood group antigens. On samples graded according to the revised working formulation for lung allograft rejection, we found chimeric macrophages (73.1 to 87.2%) in all cases and chimeric endothelial cells (1.3 to 2.1%) in four patients. Another method using ABO blood group also showed endothelial cells positive for recipient-type blood group antigens in three patients. By both methods, presence of chimeric endothelial cells was related to pathological signs of acute rejection (P<0.05).