The investigators' preliminary studies demonstrated that repetitive transcranial magnetic stimulation (rTMS) of the brain may determine a slight slowing in the rate of disease progression in ALS patients (Di Lazzaro et al 2004, 2006). The aim of this study is to investigate whether rTMS of the motor cortex performed over a long period of time (12 months) in a group of patients with ALS, can have a more pronounced beneficial effect. The investigators will compare the disease progression in two groups of patients: the first group of patients will be treated with real rTMS (one week daily treatment every month) and the second group of patients will be treated with sham (placebo) rTMS.