The twins have been surveyed periodically over the past two decades, most recently in 1985. Information obtained included questions on vascular risk factors, myocardial disease, and stroke. An analysis was made on an estimation of heritable risk. A comparison was made between monozygotic (MZ) and dizygotic (DZ) twins. If genes influenced the prevalence of stroke, there should have been more MZ twin pairs with stroke. Measures of concordance and heritability were used to estimate the size of the genetic contribution. Secondary analyses focussed on compound risk factors (e.g. hypertension and diabetes), less well documented risk factors (e.g. diet and personality), and the heritability of individual stroke risk factors.