BACKGROUND: Stroke is a leading cause of death and long-term disability worldwide.Although a minority of ischemic strokes in the community affect younger adults, up to 40% ofacute ischemic strokes in young adults are cryptogenic in nature, that is, no cause is determined.Underlying pathologies of stroke of unknown cause are multiple, including patent foramen ovale(PFO). The PFO is the most common defect of atrial septum of the heart. This study evaluatedthe frequency of PFO in brain stroke with unknown etiology in patients younger than 50 years ofage in Kerman.METHODS: This cross-sectional study was done in Shafa Medical Center of Kerman Universityof Medical Sciences in 2008. For detection of the PFO, we used agitated saline test withtranscranial Doppler sonography in brain stroke patients with unknown etiology and also acontrol group (normal persons).RESULTS: PFO was found in 53% of patients. No significant difference was observed betweensexes. The rate in the control group was 20%. Patients with large PFO had 2 or more attacks ofstroke. Subjects in the control group did not have large PFO.CONCLUSION: One of the most important underlying causes in young adults with cryptogenicstroke is PFO. It is better to prescribe antiplatelet drugs in patients with the first attack ofstroke, but as for patients with recurrent stroke, closure of PFO must be considered.Keywords: Stroke, Young Adults, Patent Foramen Ovale, Agitated Normal Saline Test.