Nitric oxide (NO) is a nonspecific inflammatory mediator that has been involved in the pathogenesis of multiple sclerosis (MS). The influence of the promoter polymorphism of inducible NO synthase gene ( NOS2A) on susceptibility and outcome was studied in 140 MS Spanish patients and 147 healthy controls matched for sex, age, and ethnicity. No association was found between MS susceptibility, course or outcome of the disease, and NOS2A polymorphisms.