The hemodynamic findings related to the histologic changes in the small pulmonary arteries in congenital heart disease were analyzed with a left-to-right shunt. A lung biopsy was performed during the repair because of pulmonary arterial (PA) hypertension (mean PA pressure ≥ 15 mmHg) in 38 patients. There were 13 patients whose age ranged from 2 years to 25 years old. A preoperative cardiac catheterization was performed to locate the site of the defect and to determine the preoperative hemodynamic findings. Among them, 29 patients had pulmonary vascular resistance (PVR) > 2.5 unit/m². The lung biopsy specimens were investigated microscopically for the Heath-Edward grade, morphometric analysis of medial wall thickness (MWT) and the rate of the decrease in the pulmonary arteriolar concentration (PAC) obtained by the alvoelo-arterial ratio divided by patients' age. All patients were in the Heath-Edward grade I to III (29 patients in grade I). This grade correlated with the MWT, but did not correlate with a decrease in the PAC. The MWT and the rate of the decrease in the PAC did not accompany each other, but either one had the tendency dominate the pattern in individual patients. The MWT had a close correlation with the mean PA pressure and PVR, and an even closer correlation in patients with a high PVR and those older than 2 years of age. The rate of the decreased in the PAC showed a weak correlation with the shunt volume in patients over 2 years of age or with a large shunt. In the high flow group (PVR < 2.5 unit/m², Qp/Qs > 2.0, n=14) the MWT was significantly thinner and the rate of the decrease in the PAC was significantly higher than the high resistance group (PVR > 2.5 unit/m², Qp/Qs2 < 2.0, n=13). The rate of the decrease in the PAC correlated with the patients' age, but the MWT did not. The lung biopsy results in patients who had both left-to-right shunts and pulmonary hypertension showed that the rate of the decrease in the PAC was weakly related to the shunt volume and the MWT was related to the PA pressure and PVR. Either an increased MWT or the rate of the decrease in the PAC tended to dominate. These phenomena were prominent in patients older than 2 in whom a wide range of individual variations were noted in the morphometric pattern. The medial hypertrophy and the rate of the decrease in the PAC may be induced by different stimuli or that medial hypertrophy may play a role in preventing PAC decrease.