Two groups of dogs underwent light and electron microscopic examination of both lungs up to five years after left lung autotransplantation. In one group of four dogs that had undergone ligation of the right pulmonary artery at the time of left lung autotransplantation, no generalized or consistent lesions were present in the transplant except for slight distension of net capillaries and slight capillary basement membrane thickening. Aside from theseminor changes which were probably of no functional significance, most areas of lung examined up to five years after transplantation were normal. In the second group of four dogs that did not have contralateral pulmonary artery ligation at the time of left lung autotransplantation, no consistent abnormality was present in either the transplanted left or nontransplanted right lung. In three animals in this group, widespread normal areas were present bilaterally up to five years after transplantation. One animal in this group had focal changes consistent with chronic pulmonary disease in both of its lungs. Thus, transplanted lungs do not necessarily develop significant late pathologic lesions.