Nitric oxide has been associated with insulin resistance and type 2 diabetes mellitus (DM). An association has been suggested between a single nucleotide polymorphism (Glu298Asp variant) of the endothelial nitric oxide synthase (eNOS) gene and increased risk of DM. However, the role of this polymorphism in favouring DM has not been investigated in hypogonadism, in which low testosterone and obesity are believed to play the major role. We aimed to evaluate whether eNOS gene single nucleotide polymorphism (Glu298Asp variant) might give a relevant contribution also to the onset of hypogonadism-associated DM. 110 men affected by late-onset hypogonadism were retrospectively reviewed. Patients were clinically and biochemically evaluated. Detection of eNOS Glu298Asp polymorphism was performed. After splitting the sample according to the three genetic variants (i.e. eNOS(GG), eNOS(GT), eNOS(TT)), no difference was evident in age, body mass index (BMI) and total testosterone. Conversely, DM prevalence, glycaemia and glycated haemoglobin were significantly higher in eNOS(TT) than in eNOS(GT) and eNOS(GG). Logistic regression analysis showed that, after adjustment for age, BMI and total testosterone, eNOS(TT) was positively and significantly associated with DM. Our study suggests that Glu298Asp single nucleotide polymorphism of the eNOS gene may be an independent risk factor for hypogonadism-associated type 2 DM.