Three cases of paradoxical embolism are presented. Although most systemic emboli originate in the heart, detection of other sources is aided by venography of the lower extremities, indocyanine green-dye curve studies of the inferior vena cava, obtaining right heart pressures, oximetry, and pulmonary angiography. Forty per cent of 55 patients reviewed had repeat episodes of systemic embolism and 2 of the 3 proved cases of paradoxical embolism had repeat episodes. Prolonged anticoagulant therapy may prevent recurrence.