Questionnaires were used to obtain information from each patient on known risk factors and diet. Fasting blood samples were obtained for lipoprotein analysis. Standard clinical and cardiological information and fasting blood samples were also collected for the cases. Blood was drawn for the DNA studies. A determination was made as to whether the Pst I Apo A-I gene polymorphism was associated with diminished levels of plasma Apo A-I or HDL cholesterol, by analysis of the distribution of these variables in cases and controls, after controlling for other known risk factors. Linkage analysis was used to determine whether the Pst I Apo A-I gene polymorphism co-segregates with premature coronary disease, or with diminished levels of plasma Apo A-I, or HDL cholesterol in 50 kindreds. A characterization was made of the abnormality in the Apo A-I, Apo C-III, Apo A-IV gene complex in patients with HDL deficiency, premature coronary disease, and the Pst I Apo A-I gene polymorphism.