Right-to-left shunt due to patent foramen ovale is a well-established risk factor for stroke in the young. The magnitude of shunt seems to be correlated to the risk of stroke in individuals. We report the cases of two 51-year-old identical twins, with similar risk factors for ischemic stroke, in which the sibling with a large and permanent shunt suffered a left hemispheric stroke and the other, with a small and latent shunt, was asymptomatic. In a three-year follow-up, the siblings were both asymptomatic, and the dimensions of shunts were unchanged. Our cases stress the importance of quantifying right-to-left shunt in order to stratify the risk of stroke in individuals, and suggest a role of heredity in patency of foramen ovale.