BACKGROUND: Echocardiography is a very important test to determine the cause of stroke which remains unidentified by routine diagnostic testing in about 40% of patients. Patent foramen ovale (PFO) is one possible cause of embolic stroke, which enables right-to-left intracardiac shunting. An association between the presence of PFO and cryptogenic stroke has been reported, but there is no report of Korean patients. METHODS: We prospectively examined 373 consecutive patients who had had a stroke, whose cause was classified according to TOAST (Trial of ORG 10172 in Acute Stroke Treatment) criteria. We compared the 88 patients with cryptogenic stroke and the 282 patients with known causes. We examined the prevalence of PFO using transthoracic echocardiography (TTE) with harmonic imaging (HI). If agitated saline bubbles were seen in LA within 3 heart beats after opacification of right heart, PFO was diagnosed. RESULTS: The prevalence of PFO was greater in patients with cryptogenic stroke than in patients with known causes but the difference did not reach statistical significance (14.8% vs. 8.2%, p=0.06). Among echocardiographic parameters, the size of left atrium (LA), LA volume, and LA volume index were significantly smaller in patients with cryptogenic stroke than in patients with known causes (35.5 짹 5.8 vs. 37.7 짹 7.3 mm, p=0.004, 34.6 짹 14.5 vs. 41.6 짹 21.9 ml, p=0.018, and 22.4 짹 8.6 vs. 26.5짹14.1 ml/m2, p=0.025, respectively). LIMITATIONS: The prevalence of PFO of our study was lower than other articles (about 20% in general population). Because we used TTE not transesophageal echocardiography to evaluate the patients, and it was not possible to get optimal position in many patients due to their clinical conditions, the prevalence could be underestimated. CONCLUSION: Higher prevalence of PFO in cryptogenic stroke was reported in out study, but larger ongoing study is suggested to be needed to prove statistical significance.