We investigated the association of endothelial nitric oxide synthase ( NOS3 ) polymorphisms − 786T>C, 27-bp repeat 4b/4a, and Glu298Asp with preeclampsia (PE). This was a case–control study involving 345 unrelated Tunisian women with PE and 289 unrelated age- and ethnically matched control women. The − 786C allele was significantly increased in PA patients when compared to healthy controls ( P = 0.015). In contrast, MAF of Glu298Asp ( P = 0.103) and 4b/4a ( P = 0.168) were not significantly different between the study groups. Higher frequencies of heterozygous Glu298/298Asp and homozygous − 786T/− 786T genotypes were seen in PE cases compared to healthy subjects. The combination of genotypes 221 (− 786T>C, Glu298Asp, 4a/4a) was more in PE cases compared with control women (17.68% vs. 8.36%; P = 0.029). Multivariate regression analysis confirmed this association. Genetic variation at the NOS3 locus represents a genetic risk factor for increased susceptibility to PE. [ABSTRACT FROM AUTHOR]