The perfusion defects of 70 patients with pulmonary embolism were studied with serial lung scans for periods of two weeks to 16 months. One third of 34 patients with small emboli returned to a completely normal scan pattern—on the average, ten days after the onset of symptoms. Of 14 patients with medium-sized emboli, three (21%) returned to normal (average, 18 days); of 22 patients with large emboli, four (18%) recovered normal blood flow patterns (average, 23 days). All but 18 of the 70 patients recovered at least one half of the perfusion initially absent. Of patients less than age 40, 12 (57%) recovered normal perfusion. No patient than age 60 returned to a normal scan pattern and less than one half had significant scan improvement. In one of 19 patients with significant heart disease, normal perfusion was restored; eight showed significant improvement.