Paraoxonase (PON1) is a protein component of high-density lipoprotein (HDL) particles that protects against oxidative damage to both low-density lipoprotein and HDL and detoxifies organophosphorus pesticides and nerve agents. A wide range of expression levels of PON1 among individuals has been observed. We examined the promoter region of PON1 for genetic factors that might affect PON1 activity levels. We conducted a deletion analysis of thePON1promoter region in transient transfection assays and found that cell-type specific promoter elements for liver and kidney are present in the first 200 bp upstream of the coding sequence. Sequence analysis of DNA from a BAC clone and a YAC clone identified five polymorphisms in the first 1000 bases upstream of the coding region at positions −108, −126, −162, −832 and −909. Additionally, the promoter sequences of two individuals expressing high levels of PON1 and two individuals expressing low levels of PON1 were analysed. The two polymorphisms at −126 and −832 had no apparent effect on expression level in the reporter gene assay. The polymorphisms at position −909, −162 (a potential NF-I transcription factor binding site) and −108 (a potential SP1 binding site) each have approximately a two-fold effect on expression level. The expression level effects of the three polymorphisms appear not to be strictly additive and may depend on context effects.