One thousand randomly selected individuals were given a complete physical examination. From those 1,000, 100 probands with gallbladder disease were selected. Approximately 1,200 first degree relatives of the probands were invited to participate in the study and given a physical examination. Additionally, 100 control individuals, their spouses and first degree relatives were followed longitudinally for five years. Blood samples were obtained for determination of total cholesterol, LDL cholesterol, HDL2 and HDL3 cholesterol, and genetic markers. Data were collected on age, sex, body mass and fat distribution, diet, smoking, and alcohol consumption. Annual follow-up assessed the dynamics of changes in apolipoprotein levels and profiles and those factors which contributed to their change. The random and longitudinal data were combined with pedigree data to determine the effects of major genes, polygenes, and the environment on the variability of each apolipoprotein and on the relationship between these and other risk factors. An assessment was made in 100 individuals of the contribution of genetic variability at the DNA level in determining apolipoprotein levels.