Paradoxical embolism: an underestimated entity. A plea for comprehensive work-up. Forty-one cases of arterial embolism were reviewed. The work-up included M + 2D echocardiography in 29 patients (71%), arteriography in 22 (54%), both echocardiography and arteriography in 19 (46%), and abdominal aortic ultrasound in 18 (43%). The sources of emboli were probable cardiac (8 = 20%)--mural cardiac thrombus detected by echocardiogram; possible cardiac (12 = 29%)--arrhythmias or other cardiac pathology detected without mural thrombus; probable arterio-arterial (7 = 17%)--proximal arterial thrombus detected; probable paradoxical embolism (2 = 5%)--fulfills the Johnson criteria with cardiac defect and right-to-left shunt detected by contrast echo in one patient and cardiac catheterization in the other; possible paradoxical embolism (3 = 7%)--meets two of three Johnson criteria without evidence of other source; and unknown source (9 = 22%)--conventional work-up negative or incomplete. Five of nine patients (56%) less than 50 years old had probable or possible paradoxical embolism, while in two patients (22%), the origin was unknown. Conclusion: (1) A significant proportion of patients with an arterial embolus are discharged with the source of emboli unknown, (2) paradoxical embolism must be considered and contrast saline or transesophageal echocardiogram should be done in patients under 50 years old. 