The aim of this study was to facilitate the early diagnosis of acute rejection after single-lung transplantation based on pulmonary hemodynamic findings. A Doppler flowmeter was placed in the ascending aorta and the pulmonary artery of adult mongrel dogs after single-lung transplantation. The pulmonary hemodynamics of the lung graft were then evaluated during the early postoperative period and during subsequent rejection. Twenty dogs were divided into three groups. Group 1 consisted of 6 dogs that underwent autotransplantation of the left lung. Group 2 was made up of 6 dogs that underwent allotransplantation of the left lung without immunosuppressant therapy. Group 3 consisted of 8 dogs that underwent allotransplantation of the left lung and were treated with 10 mg/kg of cyclosporine and 4 mg/kg of azathioprine. Pulmonary hemodynamics and chest roentgenograms were studied for more than 2 weeks postoperatively. Open lung biopsy was performed in some dogs to obtain graft specimens for histologic examination. The left pulmonary artery flow rate (percentage of pulmonary graft blood flow to cardiac output) decreased slightly after operation in group 1, and decreased to 14.4% after 1 week and to zero on postoperative day 10 in group 2. The pulmonary vascular resistance of the grafts in group 2 also increased exponentially. In contrast, the left pulmonary artery flow rates decreased to 29.1% on the day after operation in group 3, but recovered to 38.5% on postoperative day 14. Within a mean of 3.7 days of immunosuppressant discontinuation, the left pulmonary artery flow rates decreased to less than 12.7%, with a marked increase in pulmonary resistance.(ABSTRACT TRUNCATED AT 250 WORDS)