Lung biopsy specimens of 20 patients (aged 6 months to 29 years) with isolated ventricular septal defect (VSD) and various degrees of pulmonary hypertension were obtained at operation. The cross-section area of the pulmonary arteries was measured by a morphometrical method in each biopsy specimen and the grade of the hypertensive pulmonary vascular disease was determined according to the Health and Edwards classification. These findings were correlated to the pulmonary-to-systemic pressure ratio, to the pulmonary-to-systemic resistance ratio, and to the pulmonary-to-systemic flow ratio. The thickness of the media of pulmonary arteries (measured as area ratio) was well correlated to both the systolic blood pressure ratio and the resistance ratio resulting in a product moment correlation of r = 0.73. No direct relationship was found between the hypertrophy and the pulmonary-to-systemic flow ratio. Our quantitative morphometric data were well correlated with the qualitative Health and Edwards classification. Medial wall thickness was not significantly higher in grade III when compared to grade II. The mean values of the hemodynamic parameters were not significantly different in the Health and Edwards grade II and grade III groups. Our results indicate that the calculation of the mean pulmonary pressure and of the pulmonary-to-systemic resistance ratio are of no higher predictive value than the systolic pressure ratio. The classification of Health and Edwards appears to the sufficient for clinical evaluation.