Beginning in 1983 the investigators expanded the collection of computerized detailed family histories from the families of high school students participating in the Health Family Trees, a required health education course. Using the Health Family Trees, siblings were identified in which two or more siblings had early coronary heart disease. The information from the Health Family Trees was validated by contacting affected relatives and by collecting hospital data. These individuals then attended clinic screening. Detailed biochemical analyses of blood samples from these individuals were compared to find abnormalities that occured in both siblings with coronary heart disease and to identify specific subtypes of inherited early coronary disease. Testing for genetic linkage of DNA markers for apolipoproteins with these specific abnormalities was also done. Healthy age-sex matched controls with at least three siblings and no coronary heart disease in siblings or parents served as controls.