Familial hypercholesterolemia is a metabolic disorder inherited as an autosomal dominant trait characterized by an increased plasma low density lipoprotein (LDL) level. It has been demonstrated that the disease is caused by several different mutations in the LDL receptor gene. Although early identification of individuals carrying the defective gene could be useful in reducing the risk of atherosclerosis and myocardial infarction, the available techniques for determining the number of the functional LDL receptor molecules are not sufficiently accurate. The recent isolation of the LDL receptor gene now makes it possible to use restriction fragment length polymorphisms to study the inheritance of the defective allele in families with familial hypercholesterolemia. In the present study, we report the use of a Pvu II restriction fragment length polymorphism to follow the inheritance of familial hypercholesterolemia in a total of 79 patients from 37 different families. This restriction fragment length polymorphism allowed unequivocal diagnosis in 32.5% of the cases. Furthermore, in the Italians studied, the absence of a polymorphic Pvu II cutting site (P1 allele) was found to be strongly associated with familial hypercholesterolemia.