Objective To examine the association of intron 4 polymorphism in endothelial nitric oxide synthase ( eNOS) gene with diabetic nephropathy and its distribution in different ethnic groups of Singapore. Methods 258 patients with diabetic duration longer than 10 years (150 Chinese, 71 Malay and 37 Indian) were selected from both inpatients and outpatients of National University Hospital. Polymorphism was identified by DNA extraction, PCR, Cloning and sequencing. Results All published studies identified that the 27bp repeat polymorphism in intron 4 was biallelic ( a and b alleles ). Our study found a third allele-allele c, with six 27bp repeats. It was also demonstrated that there was no significant difference in overall allele distribution among the three main ethnic groups and the intron 4 polymorphism didn't show any association with diabetic nephropathy. Conclusion The intron 4 polymorphism was triallelic. We failed to demonstrate any association between this polymorphism with diabetic nephropathy.