ALS is a devastative disorder characterized by motor neuron degeneration. Median survival is 3 years after onset, but may vary from a few months to more than 30 years. Various factors have been suspected to play a role in such a variation, but recently, it has been described that regulatory T-lymphocytes (T regs) may mediate ALS progression and survival. Vitamin D is an hormone know to regulated T reg function in vivo and in vitro. It have recently demonstrated that vitamin D (VD) levels correlated with ALS prognosis. The investigator want to go further in the study of the immune processes that could modulate prognosis in ALS. This could allow proposing VD as a potential treatment of ALS in a future trial. More largely, this could reinforce arguments in favor of an immune intervention to attenuate the severity of this devastating disorder.