It was the goal of this study to see if relatively noninvasive CT studies could provide a quantitative index of acute lung rejection in single lung transplantation. Using volume scanning fast CT, the change in cross-sectional area of the major pulmonary arteries from systole to diastole, regional lung perfusion and ventilation was measured in 12 dogs with left lung allotransplantation before and during rejection and four dogs with left lung autotransplantation. All dogs were anesthetized and scanned in a fast computed tomography scanner (dynamic spatial reconstructor - DSR) during several ventilatory cycles and again during injection of contrast medium into the right atrium. There was significant reduction of regional air content, ventilation, perfusion and pulmonary artery compliance during rejection of the transplanted lung. The severity of these changes related linearly with the histological indices of rejection. It is concluded that minimally invasive dynamic CT imaging of transplanted lung can be used to detect acute rejection and its severity.