OBJECTIVE--To investigate the effect of successful single lung transplantation on pulmonary haemodynamic variables and right ventricular function. DESIGN--Pulmonary haemodynamic variables and right ventricular function were measured at right heart catheterisation after single lung transplantation. The results were compared with the preoperative pulmonary haemodynamic variables measured at the time of assessment for transplantation. SETTING--A tertiary referral centre. PATIENTS--Five survivors of single lung transplantation performed for end stage lung disease. INTERVENTIONS--Cardiac catheterisation in all five patients at a mean of 18 months postoperatively. Preoperative catheter data were available for comparison in four. Right heart pressures and cardiac output were measured and right ventricular angiography was performed. Perfusion scans performed for clinical reasons were used to assess the percentage of cardiac output passing through each lung. MAIN OUTCOME MEASURES--Right heart pressures, cardiac output, right ventricular function, percentage perfusion to lungs. RESULTS--After operation mean peak right ventricular pressure fell from 53 mm Hg to 33 mm Hg, mean pulmonary artery pressure from 33 mm Hg to 18 mm Hg, total pulmonary resistance from 11.2 U x m2 to 5.8 U x m2, and pulmonary arteriolar resistance from 8.9 U x m2 to 3.6 U x m2. Pulmonary artery wedge pressure and cardiac index were unchanged. Right ventricular function improved in all patients. The transplanted lung received most of the cardiac output. CONCLUSION--In patients with moderate pulmonary hypertension and right ventricular dysfunction secondary to end stage fibrosing lung disease single lung transplantation was followed by an improvement in pulmonary haemodynamic variables and right ventricular function.