The objective was to investigate whether the functional polymorphism of intron 4 in the endothelial nitric oxide synthase (eNOS) gene is associated with susceptibility to systemic lupus erythematosus (SLE) and its clinical features. The 27-bp repeat polymorphism in intron 4 of the eNOS gene was determined by polymerase chain reaction in 88 SLE patients and 95 healthy control subjects. Clinical manifestations were analysed in each patient and correlated with the genotypes. The genotype distribution of the intron 4 of the eNOS did not differ between SLE patients and control subjects (aa, ab, bb genotypes 0, 15, 73 versus 2, 19, 74 controls respectively, chi-squared = 2.21, 2 df, P = 0.331). In the lupus patients according to the intron 4 genotypes of the eNOS, there was no clinically significant difference in age at onset, anti-dsDNA titre, C3, C4 level, SLEDAI, SLICC/ACR Damage Index, or autoantibodies such as RF, anti-Ro, La, RNP, Sm, or phospholipid antibodies. However, renal involvement was higher in patients with ab genotypes than in those with bb genotypes (53% versus 26%), but it did not reach statistical significance (P = 0.062). Logistic regression showed that having the ab genotype was a significant risk factor for the development of lupus nephritis (odds ratio = 3.28, 95%CI: 1.04-10.2, P = 0.04). In conclusion, our data show that the eNOS ab genotypes may be associated with the development of lupus nephritis, suggesting individuals who carry the 'a' allele are more susceptible to lupus nephritis than those with the 'b' allele.