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In 34 patients operated on the morphological and morphometric patterns of the pulmonary vascular bed on lung biopsies, performed during the operation for ventricular septal defect or complete atrio-ventricular septal defect were studied. These patterns were related to the preoperative hemodynamic data. Ages ranged from 2 to 20 months. There were 16 ventricular septal defect (mean age 10 +/- 4 months) and 18 complete atrio-ventricular septal defect (mean age 9.8 +/- 1.2 months). Furthermore, of the latter 15/18 (83%) patients had Down's syndrome. The following hemodynamic data were considered: a) ratio between the pulmonary systolic artery pressure and the systemic systolic artery pressure; b) diastolic pulmonary artery pressure; c) pulmonary vascular resistance; d) ratio between pulmonary and systemic flow. The lung specimens obtained at surgery were routinely processed and embedded in paraffin. Sections measuring 4 microns were stained with hematoxylin-eosin, Masson's thricrome and Miller's elastin. The status of the small pulmonary arteries was assessed according to the Heath-Edwards classification and to the morphometric parameters proposed by Rabinovitch and coll. Ratio between the pulmonary systolic artery pressure and the systemic systolic artery pressure ranged from 0.48 to 1 (mean 0.8 +/- 0.23) in patients with ventricular septal defect, whereas in complete atrio-ventricular septal defects it ranged from 0.42 to 1 (mean 0.86 +/- 0.21). The mean diastolic pulmonary artery pressure was 20 +/- 10 mmHg (range 8-40 mmHg) in ventricular septal defects and 19.5 +/- 2.3 mmHg (range 10-35 mmHg) in complete atrio-ventricular septal defects.(ABSTRACT TRUNCATED AT 250 WORDS)