Paraoxonase (PON) is a serum enzyme with a wide species distribution. It protects lipoproteins from toxic oxidative modifications and is an antiatherogenic mechanism of major potential. Activity levels of PON are major determinants of the protective function; consequently, factors that influence PON levels are of particular relevance. The present study has identified 3 polymorphisms in the promoter region of the human PON1 gene. Cell transfection studies have revealed their variable impact on promoter activity, with up to 2-fold differences in reporter gene expression. Genotyping studies have established that the polymorphisms are frequent in the population, a finding that is consistent with a major impact on PON concentrations. The physiological relevance of the polymorphisms was underlined by showing that they are associated with highly significant differences in serum concentrations and activities of PON. The study thus firmly establishes a genetic basis for variations in serum PON levels and, consequently, serum PON activity. It is consistent with the suggestion that variations in a major antioxidant function of high density lipoprotein are, to an important degree, genetically determined.