This study shows that 33.3% of English patients with primary hyperlipidemia (52/156) had the S2 allele of the apo AI/CIII/AIV complex compared to 6.1% of normolipidemic individuals (3/49). The increased frequency of the allele was statistically significant in each of the hyperlipidemic groups (type IIA, excluding patients with FH, type IIB and IV) examined and was not specifically related to hypertriglyceridemia. This finding may account for the result of several studies which showed groups of patients with CHD had a significantly higher prevalence of the S2 allele than control groups. Our data do not support the notion that the increased frequency of this allele in CHD patients is independent of variations in plasma lipid levels, since we find the frequency of the S2 allele in an apparently healthy hyperlipidemic group of patients is very similar to a hyperlipidemic group with symptomatic premature atherosclerotic disease. This study also shows the BMI of the type IIB and IV hyperlipidemic patients is significantly higher than the type IIA (no xanthomas) group. This may modulate the expression of the defect associated with the S2 allele. When the type IIB and IV hyperlipidemic groups were divided into 2 groups according to their apo AI/CIII/AIV genotype (i.e., S1S1, S1S2 (including S2S2] there was no significant difference in the mean plasma level of total cholesterol, HDL-cholesterol and triglycerides between the 2 groups. In contrast the S1S2 type IIA individuals had higher plasma cholesterol levels.