From January 1971 to March 1973 all twin pairs in the Swedish Twin Registry below the age of 70, who became death-discordant, were continuously recorded. A total of 205 (78%) of the surviving cotwins were examined with respect to different manifestations of ischemic heart diseases (IHD) and several “environmental” and “biometric” risk factors. Among the death-discordant pairs, the cause of death was IHD in 57 pairs and other than IHD in 148 pairs. Analyses revealed that the prevalence rate of myocardial infarction, angina pectoris, pathologic Q-wave, and ST depressions in connection with exercise, were significantly higher among the surviving cotwins whose partners had died from IHD than those whose partners had died from other causes. The same trends were seen for most of the risk factors measured both singly and in combination, although not very pronounced. The results indicate a substantial genetic influence in the development of IHD. The genetic influence is possibly transmitted not only through some of the risk factors measured, but also through other factors, still unknown.