To evaluate the applicability of single lung transplantation as a treatment of pulmonary hypertension, we investigated the relationships between the pulmonary blood flow and resistance in allografts 10 days after single lung transplantation in dogs. In eight surviving recipients and five healthy dogs (control group), pulmonary flow-resistance relationships were investigated at flows in the range of 0.3 to 2.0 L/min at 0.1 L/min increments. Resistance decreased from 2585 to 746 dyne.sec.cm-5 in the successful group (five allografts receiving complete studies) and 2400 to 891 in the control group with an increase of flow. Lung water content of the successful group was 2.0 times that of the control group (successful group, 95.6 +/- 16.7 gm; control group, 47.8 +/- 7.5 gm), and water content of the other allografts (unsuccessful group), including allografts of dogs that died before investigations, was 2.2 times that of the successful group (unsuccessful group, 211.0 +/- 89.6 gm). The chest roentgenograms of the successful group showed mild or moderate consolidations, whereas those of the unsuccessful group showed severe consolidations, but no difference was found in pulmonary flow-resistance relationships between the successful and the control groups. These results show that the allografts, which have about twice the water content of normal lung, maintain adequate pulmonary vascular function. We concluded that single lung transplantation could be an effective treatment for pulmonary hypertension.