The natural history of pulmonary vascular obstruction in VSD associated with PH was studied by measuring PVR before and after injection of tolazoline 1 mg/kg. Organic pulmonary vascular obstruction (PVO) was minimal in infancy, but increased linearly progressively with advancing age, and average rate of increase in PVR after tolazoline was 1 unit.m2/year. The rate of increase in PVR after tolazoline in Eisenmenger complex was variable and ranged between 0.7 to 4 unit.m2/year in most cases. Analysis of 7 cases of post-operative late death due to PVO revealed that usually these cases were over 5 years of age at operation, and pre-operative PVR was more than 12 unit.m2. Reduction of PVR was confirmed post-operatively in most of these cases whose pre-operative PVR before and after tolazoline were les than 8 unit.m2 and 7 unit.m2 respectively. Early surgical repair is indicated in VSD and PH to prevent progress of PVO.