Transesophageal echocardiography has been described as superior to transthoracic echocardiography in the detection of patent foramen ovale (PFO). Utilizing both methods, we were able to determine the frequency of PFO in younger adult patients with ischemic stroke (patient group) and in age-matched control subjects (control group). Thirty-four patients under 55 years of age with cerebral or retinal ischemic attacks (mean age 39 years) and 40 controls (mean age 39 years) were studied. Transesophageal echocardiography detected a significantly higher frequency of PFO in the patient group (44%) than in the control group (18%, p less than 0.05). In comparison with transthoracic echocardiography, transesophageal echocardiography found a higher frequency of PFO in both the patient group (12% vs 44%, p less than 0.01) and the control group (3% vs 18%, p less than 0.05). All of the foramina proved to be patent by transthoracic echocardiography were also found by transesophageal echocardiography. The results suggest that transesophageal echocardiography is a sensitive method for demonstrating PFO. Paradoxical embolism through an interatrial communication (PFO) may be a possible cause of strokes in the young adult.