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Among 240 consecutive patients subjected to total hip replacement, 206 underwent perfusion scintigraphy in screening for pulmonary embolism 10 to 14 days after surgery. All patients with perfusion abnormalities (n=77) also underwent ventilation scintigraphy with a dry 99Tcmmicroaerosol. The scintigraphic abnormalities were classified according to a detailed coding system and in 50 patients pulmonary angiography was performed within 24 hours for correlation. Emboli were diagnosed in 19 patients at angiography. All patients with ventilated segmental (n=6) or larger (n=5) perfusion defects or multiple subsegmental defects larger than half a segment (n=2) had emboli. Two of 8 patients with abnormally ventilated subsegmental perfusion defects larger than half a segment had emboli. Four of 21 patients with wedge-shaped subsegmental perfusion defects smaller than half a segment or defects with a rounded border towards the hilum and a peripheral extension equal to or less than that of a segment had emboli. In one of them the site of the emboli correlated to the site of the perfusion defect but occurred in normally perfused areas in the remaining 3. Ventilation scintigraphy was of no value in this group of patients. No emboli were found in patients who had areas with decreased but not totally lost perfusion (n=8). No emboli were diagnosed in 27 lungs with a normal perfusion.