Morphological changes of the bronchus and lung tissue of ten adult dogs were examined at various intervals after sleeve resection of the left upper lobe was performed in combination with bronchoplasty and pulmonary artery angioplasty. Postoperative changes in the bronchus and pulmonary artery were investigated by bronchoscopy and pulmonary angiography 8 months to 14 months after the operation. The dogs were sacrificed 9 months to 32 months after the operation, and the bronchus and lung tissue were submitted for microscopic and scanning electromicroscopic investigation. The bronchial mucosa at the anastomosis showed irregular height of the mucosal epithelial cells, the continuity of which was smooth with normal cilial appearance. The peripheral lung tissue showed no obvious difference between the operated lobe and the non-operated one. Dilatation of the internal diameter of the bronchial anastomosis was seen in 3 dogs in which the bronchial cartilage showed a degenerative change at the anastomosis. These changes were thought to be related to the anastomosis method and suture tension. Therefore it was suggested that much attention was paid to the bronchial anastomosis. Absorbable suture material, Dexon, was well absorbed and showed no foreign-body or inflammatory reaction. Based upon these results, excellent healing can be expected after sleeve resection despite some degenerative changes, and the clinical application of this procedure seems acceptable.