OBJECTIVE. The purpose of this study was to correlate scintigraphic findings of regional alterations in lung ventilation and perfusion with regional variations in CT attenuation in patients with primary pulmonary hypertension. SUBJECTS AND METHODS. Chest CT scans and ventilation-perfusion scans obtained within 24 hr of each other in 18 patients with primary pulmonary hypertension referred for lung transplantation were reviewed. The lungs were divided into eight regions (left/right, superior/inferior relative to the carina, and anterior/posterior relative to the trachea). CT scans were evaluated and areas of parenchymal inhomogeneities were tabulated for the eight regions. Areas of reverse mismatch (perfusion without ventilation) were established by blinded analysis of planar scintigraphic studies in six projections using 99mTc-labeled DTPA-aerosol and macroaggregated albumin for the eight regions and then were correlated with the CT findings. RESULTS. Abnormal findings on ventilation scans and reverse ventilation-perfusion mismatches indicating an inadequate hypoxic vasoconstriction reflex were found in 91 regions in all 18 patients. Nonuniform parenchymal CT density was found in 12 patients. There was a significant correlation (p = .009) of scintigraphic reverse mismatches with abnormal CT density in 38 regions in 11 patients. In one patient, there was no scintigraphic correlation with abnormal CT attenuation. The specificity of abnormal CT density for scintigraphic reverse mismatches was 81%, with a sensitivity of 42%. CONCLUSION. Scintigraphic reverse mismatches indicate a high prevalence of significant pulmonary arterial shunting in patients with ventilatory defects. Increased relative CT attenuation in areas of impaired ventilation as shown on the ventilation scans is amplified in primary pulmonary hypertension by an inadequate hypoxic vasoconstriction reflex. This finding does not signify underlying infiltrative long disease and correlates with regions with reverse mismatches.