The epidemiology and common risk factors for cerebrovascular disease have been well documented by cross-sectional and longitudinal population surveys. These studies have generally involved unrelated individuals with little information on family history. Despite a significant role in cardiovascular risk, few studies have investigated familial contributions to stroke risk, and its importance was unknown. Recognition of the importance of hereditary influences on vascular disease of the heart has contributed, in large part, to the current emphasis on the molecular biology of vascular disease and a more balanced view that recognized both genetic and environmental influences on coronary artery disease. Twin studies represented a simple and uniquely powerful tool for analyzing genetic and environmental contributions to complex human phenotypes. Studies of cardiovascular risk have shown the importance of heritable factors, and it was hypothesized that similar genetic factors played a role in development of stroke.