In the course of investigating familial coronary artery disease in Utah, we studied 196 members of an eight-generation extended family of familial hypercholesterolemia (FH), in which 73 members were affected with type IIa hyperlipoproteinemia (HLPIIa; high plasma cholesterol) and 11 members with type IIb hyperlipoproteinemia (HLPIIb; high plasma cholesterol as well as plasma triglyceride). A splice-site mutation of the LDL receptor (LDLR) gene (IVS14 + G > A) co-segregated with elevated plasma cholesterol among all the members, but not with the elevated plasma triglyceride and VLDL cholesterol levels seen in HLPIIb patients. The apolipoprotein H (apoH) gene plays a role in plasma triglyceride removal and lipoprotein lipase enhancement. Intra-familial correlation analysis of the modifier effect of Val247Leu substitution in the apoH gene was carried out among 84 LDLR-mutation carriers and 112 non-carriers. When plasma triglyceride levels in the LDLR-mutation carriers were compared, the values were lowest among V/V homozygotes (mean +/- SD = 145 +/- 53 mg/dl), highest in L/L homozygotes (277 +/- 177 mg/dl), and intermediate among V/L heterozygotes (191 +/- 102 mg/dl) (p = 0.0015). All eleven patients who presented with HLPIIb had inherited both the defective LDLR allele and an apoH 247Leu allele, whereas all 45 carriers of the defective LDLR allele not carrying the apoH Leu allele presented with HLPIIa but not HLPIIb (p = 0.0001). These results indicate a significant modification of the phenotype of FH with a defective LDLR allele, by apoH Leu variation in our studied family.