We performed this study to assess the correlation of residual pulmonary hypertension in the immediate postoperative period with that in the late follow-up period, to assess the histologic changes of pulmonary arteries (PA) at the time of repair for patients with congenital heart disease consisting of left-to-right shunt, and to clarify the role of lung biopsy in determining the operability and reversibility of pulmonary vascular changes. Lung biopsy was performed during repair in 38 patients, with a wide range of age, who had congenital left to right shunt and pulmonary hypertension. All were Heath-Edward grade III or less. Morphometric study included measurement of medial wall thickness (MWT) and decrease rate of pulmonary arterial concentration (PAC). Mean PA pressure in the immediate postoperative period was measured in all 38 patients. Follow-up cardiac catheterization was performed in 15 patients (average 3.8 years after repair). At operation, 5 patients of this late follow-up group were under 2 years of age and the other 10 were 2 or more. During catheterization, pulmonary hemodynamic reaction was observed both under room air inhalation and after inhalation of hypoxic gas FiO₂ 0.15. Mean PA pressure and pulmonary vascular resistance (PVR) in the immediate postoperative period had a significant correlation with PA pressure and PVR values before the operation, but not with morphometry, Heath-Edward grade, or with pulmonary hemodynamics in late follow-up. During the late follow-up study, 5 of the 15 patients had pulmonary hypertension (defined as mean PA pressure ≥ 15 mmHg) under room air inhalation, and PA hypertension was induced in 4 additional patients after hypoxic gas inhalation. There was no incidence of PA pressure or PVR values registering above the preoperative level. The degree of PA hypertension showed a correlation with the rate of PAC decrease and also with patients' age-at-operation. Multiple regression analysis showed that both the rate of PAC decrease and the age-at-operation contributed significantly to the degree of PA hypertension. Some of the patients over age 2 had a decreased rate of PAC above the regression line, which none of the patients under age 2 experienced. In patients with Heath-Edward grade III or less, residual pulmonary hypertension in the immediate postoperative period was not correlated with histology, but in late follow-up, it was with PAC and the age-at-operation. Therefore, a decrease of PAC is assumed to be a totally or partially irreversible pulmonary vascular change depending on the patient's age-at-operation, while medial hypertrophy is thought to be a reversible pulmonary vascular change. Lung biopsy could play an important role in determining the reversibility of pulmonary vascular obstruction, particularly in patients older than 2 years.