We investigated the effects of paraoxonase (PON 1) 192 polymorphism on serum PON1 activity and the impact of phenotypic expression on the risk and prognosis of Turkish children with membranoproliferative glomerulonephritis (MPGN). Eighteen children with biopsy-proven Type I MPGN (10 boys, 8 girls) and age-matched 53 healthy controls were included in the study. PCR (polymerase chain reaction), RFLP (restriction fragment length polymorphism) and agarose gel electrophoresis techniques were used to determine the PON1 192 genotype. PON1 activity was measured by spectrophotometric assay of p-nitrophenol production following addition of paraoxon. We found that PON) 192 genotype distribution (AA, AB, BB) in MPGN patients were 61.1%, 22.3%, 16.6% and 15.1%,35.8%,49.1% in controls, respectively. The frequency of AA genotypes was significantly higher in the MPGN group (0.611) compared with the healthy controls (0.151)) < 0.001). Although the serum PON1 activity was lower in MPGN patients (103.3 +/- 55.2U/1) than the healthy controls (130.9 +/- 71.2U/mol), the difference was not statistically significant (p = 0.0563). In the genotypes of patients and controls classified according to PON1 A/B polymorphism; serum PON1 activities were significantly increased (p < 0.00 1, ANOVA) in the order of PON1 AA, AB and BB in both MPGN patients (82.4, 91.7 and 173.6 U/1) and healthy controls (85.9, 119.9 and 193.1 U/1), respectively. There was a significant relationship between the poor prognosis and having AA genotype and low PON1 activity. Of the 8 patients with poor prognosis, 7 had genotype AA and the remaining one was AB heterozygote. Our results suggest that homozygosity for the A allele might have an important role on the risk for developing MPGN and may also be associated with the poor prognosis of disease. In conclusion, we suggest that the PON I activities are affected by PONI genetic variability in Turkish patients with MPGN. Copyright (c) 2005 John Wiley & Sons, Ltd.