We have identified a naturally occurring mutation in the promoter of the lipoprotein lipase (LPL) gene. One of 20 patients with familial combined hyperlipidemia (FCHL) and reduced levels of postheparin plasma LPL activity was found to be a heterozygote carrier of this mutation. The mutation, a T-->C substitution at nt -39, occurred in the binding site of the transcription factor Oct-1. As a result, the transcriptional activity of the mutant promoter was < 15% of wild type, as determined by transfection studies in the human macrophage-like cell line THP-1. This decrease in promoter activity was observed in undifferentiated as well as in phorbol ester-differentiated THP-1 cells. Furthermore, the inductive effect of elevating the levels of intracellular cAMP was equally reduced. This mutation was not present among 20 FCHL patients with normal plasma LPL levels nor has it been reported among individuals with familial LPL deficiency. Thus, heterozygosity for LPL promoter mutations may be one of several factors that contribute to the etiology of FCHL.