Thirty-six patients ranging in age from 7 months to 15 years and weighing from 5300 g to 49 kg (24 undergoing corrective surgery and 12 cases with reversed shunt and no operation) underwent technetium 99m hexamethyl propylenamine oxime (Tc-99m HMPAO) lung clearance study and the results were compared with catheterization and pathology. Patients were allocated into three groups with respect to pathological grading (Heath-Edwards' classification) and the results were correlated on the basis of pathology. In group I (grades I and II), Pearson correlation coefficient was 0.86 with pulmonary artery pressure (PAP), pulmonary vascular resistance (PVR), and Tc-99m HMPAO lung clearance (t(1/2)). Pearson correlation coefficients were 0.863 and 0.88 in the second (grade III) and third group (with reversed shunt and no operation). There were statistically significant differences among the groups with respect to PAP, PVR, or t(1/2). The results of radionuclide study (t(1/2)) were very well correlated within the groups with respect to hemodynamic parameters (PAP and PVR). Tc-99m HMPAO has potential as a highly sensitive indicator for detecting early and minimal microvascular lung injuries, and it may reflect accurate lung clearance and retention enabling an estimation of the state of pulmonary hypertension.