Objective: Diabetes mellitus is a common metabolic disorder characterized by hyperglycemia. Cytokines induce signal transduction and secondary messenger pathways after binding their specific ligands. In this way, these proteins control and also modulate activity of the target cells. Tumor necrosis factor plays an important role in diabetes and is a member of the family of cytokines. In this study, we aimed to investigate the effect of A252G (TNF-beta NcoI, rs909253) polymorphism in the tumor necrosis factor beta gene on the Type I Diabetes Mellitus in the Turkish population.