To investigate the relationship between 27 bp repeat polymorphism in intron 4 in the endothelial nitric oxide synthase (eNOS4) gene and essential hypertension in the Kazakh Chinese population, 151 patients with essential hypertension and 138 healthy people were selected from the Boertonggu countryside of Shawan region in the Xinjiang uygur autonomous region of China in 2006. The polymorphism of eNOS in the two groups was detected with polymerase chain reaction assays and the genotype frequencies in each group were calculated following the Hardy-Weinberg law. Four and five tandem 27 bp repeats were designated as "a" and "b", respectively. It was found that the frequencies of b/b, b/a and a/a genotypes of the eNOS4 gene were 84.06%, 15.22% and 0.72% in the control group, and 81.46%, 15.89% and 2.65% in the hypertension group, respectively. The frequencies of gene "a" and "b" were 91.67% and 8.33% in the control group and 89.40% and 10.60% in the hypertension group, respectively. It was found that plasma eNOS activity was not associated with genotypes and alleles of eNOS gene. Plasma eNOS activity in the hypertension group was significantly decreased compared with the control group (P<0.01). The results suggest that eNOS4 gene polymorphisms are unlikely to be the major genetic susceptibility factors for essential hypertension in the Xinjiang Kazakh population. However, a positive association between plasma eNOS activity and essential hypertension has been revealed.