Monomelic amyotrophy (MA) is a variant of motor neuron disease (MND), characterized by muscle weakness and atrophy restricted to one limb. We describe the case of a 56-year-old Italian patient who developed a segmental muscular atrophy limited to the lower left limb. After 11 years of clinical stability he developed progressive spread of the disease to all limbs and to bulbar and respiratory muscles. The patient died from respiratory failure 15 years after disease onset. This case demonstrates that monomelic amyotrophy may rarely evolve to a diffuse fatal MND, even after more than a decade of clinical stability. Our findings support the idea that MA is part of the clinical continuum of MND.