The A1/A1 genotype of the anti-inflammatory cytokine interleukin 1 receptor antagonist (IL-1Ra) polymorphism was more frequent in 339 Italian MS patients than in healthy controls (HCs) (odds ratio = 1.83). A more aggressive disease course was also associated with A1+ genotypes and might reflect the reduced ability of mononuclear cell cultures of A1+ HCs to produce IL-1Ra. We conclude that an IL-1Ra gene polymorphism is associated with occurrence of disease and clinical course variability in Italian MS patients.