Background/Aim: The phenotypic variability of autosomal dominant polycystic kidney disease (ADPKD) cannot be explained only by various mutations of two known genes (PKD1 and PKD2), but the influence of other unknown factors should also be considered. Impairment of endothelial function has been observed in ADPKD patients. Our study tried to verify the hypothesis that endothelial nitric oxide synthase (ecNOS) gene polymorphisms in intron 4 could have some relevance to the progression of ADPKD. Methods: We examined 155 Czech patients with ADPKD (80 males, 75 females; mean age 43.4 ± 14.7 years) and 100 genetically unrelated healthy subjects (50 men, 50 women; mean age 51.2 ± 8.2 years). The genomic DNA was amplified by polymerase chain reaction and the products were separated on 1.5% agarose gel and visualised by ultraviolet transillumination. We compared subjects homozygous for the ecNOS b allele with subjects homozygous and heterozygous for the ecNOS a allele. Results: The frequencies of ecNOS b/b, ecNOS a/b and ecNOS a/a were 81% (81/100), 17% (17/100) and 2% (2/100), respectively, in the control group, 60% (30/50), 32% (16/50) and 8% (4/50), respectively, in ADPKD patients with end-stage renal failure (ESRF), and 68.5% (72/105), 28.6% (30/105) and 2.9% (3/105), respectively, in ADPKD patients with normal renal function. The two-tailed t test revealed that the frequency of the ecNOS a allele (ecNOS a/b and ecNOS a/a) in dialysed patients was significantly higher than in ADPKD patients with normal renal function (p < 0.05). Conclusion: The a allele of the ecNOS gene polymorphism showed a significantly higher incidence among patients with ESRF caused by ADPKD. The a allele might have a negative influence on the progression of ADPKD.