Dendritic cells are essential for the activation of the type IV immunological reactions that are intrinsic to rejection of transplanted organs. We evaluated the number of dendritic cells in the recipient and donor portions of the trachea and donor bronchi of 6 heart-lung transplant recipients, 3 of whom had evidence of bronchiolitis obliterans, a presumed manifestation of chronic rejection of the lung. As compared with recipients without bronchiolitis obliterans, patients with it showed a significant increase in the number of S100-protein-positive dendritic cells in the tracheal and bronchial epithelium and submucosa of the donor organs. This finding of increased accessory cells in the transplanted mucosa provides support for the belief that bronchiolitis obliterans is an immunologically mediated form of chronic rejection.