Abstract BACKGROUND: Patent foramen ovale (PFO) is the most commonly persistent abnormality of fetal origin. PFO has long been recognized as a potential risk factor for ischemic stroke. This study has shown the prevalence of PFO among young patients with cryptogenic stroke. METHODS: In our case-control study we had 32 patients, 18 to 55 years old with cryptogenic stroke and 64 participants among normal population with matched age and sex in control group. We studied them for stroke risk factors like hypertension, diabetes mellitus, ischemic heart disease, dyslipidemia and then election of PFO by contrast trans-thoracic echocardiography. Data entered in SPSS11 and analyzed by Chi-Square and logistic regression. P value less than 0.05 was considered statistically significant. RESULTS: We found that 37.5 % of patients in case group and 7.7 % of patients in controls had PFO and this difference was statistically significant (P = 0.001). They had no significant difference in other atherosclerosis risk factors. In control group we saw small shunt but in stroke group large shunt was more prevalence (P < 0.05). CONCLUSION: Our findings supported this idea that PFO is a predisposing factor for stroke and it had a higher prevalence among patients with cryptogenic stroke. Besides, large shunt was more concomitant with ischemic attack. Then we suggest any patient with undefined cause of stroke must be evaluated for PFO. Keywords: Patent foramen ovale, Stroke, Young.