In patients who have suffered a stroke or occlusion of a large artery in another body part of unknown origin a possible cause is a small opening between the upper heart chambers (patent foramen ovale, it is called). After birth this opening closes in 75% of the population, while it persists in 25% of people. It may allow a small blood clot to pass from the veins of the legs through the heart into the brain or other parts of the body. In order to reduce the risk for a further attack we have today more therapeutic options to choose from but it is unclear which strategy have the best outcome. This study is created to compare the effect of two treatment strategies: