This study examines the pulmonary hemodynamics during reimplantation and rejection following unilateral lung transplantation in dogs. Nineteen mongrel dogs were divided into three groups according to the method of treatment used: group 1 (n = 6) underwent modified autotransplantation of the left lung; group 2 (n = 6), allotransplantation without immunosuppression; and group 3 (n = 7), allotransplantation with immunosuppression. The pulmonary arterial blood flow and ventilation scores, assessed by chest X-rays, were measured for 2 weeks postoperatively. Pulmonary blood flow to the transplanted lung decreased slightly in group 1, whereas in group 2, it decreased sharply from the 3rd postoperative day (POD) and was almost completely absent by the 14th POD. In group 3, it decreased by 28% on the 1st POD but recovered gradually to 37% by the 14th POD. Ventilation scores were the same among the three groups on the 1st POD but in group 2, they decreased rapidly from the 3rd POD. This change correlated well with that of the pulmonary arterial (PA) flow, although the ventilation score changed after the PA flow did. In conclusion, rejection was reflected earlier and more distinctly by the changes in PA flow than by chest X-rays.