Preeclampsia is a pregnancy-specific disorder in humans and a major cause of maternal and neonatal morbidity and mortality. Increasing evidence suggests that oxidative stress plays an important role in the pathogenesis of preeclampsia. The aim of this study was to investigate the relationship between null alleles of the glutathione S-transferases (GST) M1 and T1 genes and the risk of preeclampsia. This case-control study involved 112 preeclamptic and 233 normoevolutive pregnant women. The null polymorphisms were genotyped by multiplex polymerase chain reaction (PCR). Our results showed an increased risk of preeclampsia in patients with the GSTT1 null genotype [odds ratio (OR) = 2.21; 95% confidence interval (CI) = 1.14-4.27; P = 0.018]. Our data further showed that a combination of deletion genotypes of the GSTM1 and GSTT1 genes conferred an even higher risk of preeclampsia (OR = 4.56, 95%CI = 1.59-13.09; P = 0.005). Our results provide the first evidence suggesting that a GSTT1 null polymorphism might be associated with preeclampsia in the Mexican mestizo population, and that this risk increases with the combination of both GSTT1 and GSTM1 null polymorphisms.