Familial combined hyperlipidemia (FCHL) is a frequent cause of premature coronary artery disease. Affected family members are characterized by different combinations of elevated cholesterol and/or triglyceride levels. A reduction in lipoprotein lipase (LPL) activity has been observed in a subgroup of FCHL patients. Recently, we have demonstrated an increased frequency of mutations in the LPL gene in Dutch FCHL patients compared to normolipidemic controls. In the present study, we have applied a pedigree-based maximum likelihood method to study the effect of LPL mutations on the phenotypic expression of FCHL in families. In 40 FCHL probandi, three different previously reported mutations in the LPL gene were identified resulting in amino acid changes, D9N, N291S, and S447X. The D9N mutation in exon 2 appeared to be in strong linkage disequilibrium with a T-->G substitution at position -93 in the promoter region of the LPL gene. We present data that the -93T-->G/D9N haplotype is associated with significantly higher levels of LDL and VLDL cholesterol, and VLDL triglycerides. Interestingly, the effect was only observed in male carriers. In line with our previous observations, these results further sustain that the LPL gene is a susceptibility gene for dyslipidemia which explains part of the variability in the phenotype observed among FCHL family members.