['We investigated the relationship between the distribution of the eNOS4a/b polymorphism and the clinical features of superficial bladder cancer.', 'This study included 201 healthy controls with a mean ± SD age of 62.35 ± 7.96 years and 123 patients with a mean age of 64.03 ± 11.00 years diagnosed with histopathologically confirmed superficial bladder cancer. The eNOS4a/b polymorphism genotype (aa, bb or ab) was identified by polymerase chain reaction. Blood glutathione and plasma malondialdehyde levels were measured by spectrophotometry as an indicator of oxidative stress. We estimated total plasma levels of nitric oxide metabolites using a colorimetric assay kit.', 'There were no significant differences in age or body mass index between patients and controls. Malondialdehyde and nitric oxide metabolite levels were statistically significantly increased (p = 0.000 and 0.024, respectively) and glutathione levels were decreased (p = 0.000) in patients with superficial bladder cancer. The bb genotype of the eNOS4a/b polymorphism is the most frequent one in the Turkish population and the aa genotype was significantly more common in patients with superficial bladder cancer (p = 0.000). Also, the aa plus ab genotype was significantly more common in patients with high grade tumors (p = 0.013) and in those with more progression to muscle invasive disease (p = 0.000). This genotype was also a significant independent risk factor for recurrence after adjusting for smoking status, stage, grade and the presence of carcinoma in situ on logistic regression analyses (OR 3.095, 95% CI 1.21-7.86, p = 0.018).'] Is nitric oxide synthase ( eNOS4a/b ) gene polymorphism associated with tumor recurrence and progression in superficial bladder cancer cases?