This repository will establish for the first time a system to carefully assess and monitor over time the general health and the amount of cholesterol in the arteries of U.S. children and adults with homozygous familial hypercholesterolemia (hoFH). Patients with this very rare disorder have very high blood levels of cholesterol from birth due to the inheritance of an abnormal gene from each parent. As a result, if untreated, heart attacks and sudden death occur in childhood. Treatments such as LDL-apheresis and liver transplant will lower the cholesterol level, but the best treatment and the best way to monitor the effect of the treatment on the arteries are unknown. The collection of clinical data and blood for analysis of known and yet-to-be discovered markers and predictors of arterial disease will yield new information about the natural history of the disorder and response to treatment. The repository will greatly aid the development of specific protocols that seek to learn more about this disease and new therapies.