Cardiac death from atherosclerosis is common in hemodialysis patients. Human serum paraoxonase (PON1), an esterase, is associated with high-density lipoprotein and inhibits the susceptibility to oxidization of low-density lipoprotein (LDL). The PON1 genetic polymorphisms of 192 Gln/Arg and 55 Leu/Met in the amino acid sequence are partly involved in the PON1 enzyme activity. We investigated the PON1 enzyme activities for paraoxon (paraoxonase) and phenylacetate (arylesterase), and the two polymorphisms in 96 patients undergoing hemodialysis and in 136 normal controls. Both activities were significantly lower in the hemodialysis patients than in the controls (97+/-43 vs 155+/-57 micromol/min/l for paraoxonase, and 71+/-20 vs 92+/-22 mmol/min/l for arylesterase, respectively). There was no difference in the distribution of the two polymorphisms between patients and controls, and in every subgroup classified by the polymorphisms, both paraoxonase and arylesterase activities were lower in patients than in controls. This suggested that the enzyme activities of PON1 decreased in hemodialysis patients, independent of the genetic polymorphism. The decrease in PON1 enzyme activity in hemodialysis patients may modify a susceptibility to oxidization of LDL, which contributes to an acceleration of atherosclerosis.