The incidence of pulmonary perfusion defects after routine cardiac Catheterization was assessed in 57 patients by comparing ventilationperfusion lung scans obtained before and 1 day after Catheterization. Patients were prospectively randomized to two groups, one in which right heart Catheterization was performed using an antecubital venous cutdown procedure and one in which the percutaneous femoral vein approach was used. Seven patients (12 percent) had new postcatheterization perfusion defects consistent with pulmonary emboli. These patients did not differ significantly from patients without new defects in clinical characteristics, duration of Catheterization, hemodynamic variables or route of right heart Catheterization. The data suggest that pulmonary embolism may be a more common complication of routine cardiac Catheterization than previously appreciated. © 1979.