The behaviour of pulmonary hypertension and elevated pulmonary vascular resistance was followed up to 12 years postoperatively in 29 patients with patent ductus arteriosus (DAA) and 34 patients with ventricular septal defect (VSD). 8 of 11 patients with VSD and rest defect underwent a second operation. All patients had preoperatively pulmonary hypertension due to elevated pulmonary vascular resistance (RPA) and elevated pulmonary blood flow. Pulmonary arterial mean pressure (PPA) was usually reduced immediately after operation corresponding to the reduced pulmonary blood flow. In the later postoperative period PPA and RPA were either reduced to normal values, reduced but not normalized or increased progressively. In patients with moderate elevation of RPA before operation (200-800 dyn - sec cm-5) 34 % showed normalisation of RPA, 11 % increase of RPA in the follow-up period. In patients with severe elevation of RPA before operation (greater than 800 dyn - sec - cm-5) 11 % showed normalisation, 30 % an increase of RPA in the follow-up period. There was no correlation between age at operation and the late postoperative results. Exercise test were useful to confirm normalisation of the pulmonary vascular changes. The results in patients with VSD and rest defect demonstrate that apparently small left-t0-right shunts are able to stop the involution of organic changes in the pulmonary arteries. Prognosis of follow-up cannot be established from the hemodynamic results obtained early after operation. Therefore repeated hemodynamic investigations have to be performed in the later follow-up period.