The concurrence of amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS) is extremely rare. We reported the case of a 33-year-old woman with a past history of paresthesias at the right hand, who developed progressive quadriparesis with muscular atrophy of limbs and, finally, bulbar signs and dyspnea. Clinical and neurophysiologic investigations revealed upper and lower motor neuron signs in the bulbar region and extremities, suggesting the diagnosis of ALS. Moreover, magnetic resonance imaging (MRI) and cerebrospinal fluid (CSF) analysis demonstrated 3 periventricular and juxtacortical lesions, hyperintense in T2 and FLAIR sequences, and 3 liquoral immunoglobulin G (IgG) oligoclonal bands, consistent with diagnosis of primary progressive MS (PPMS). This unusual overlap of ALS and MS leads to the discussion of a hypothetical common pathological process of immunological dysfunction in these two disorders, although the role of immune response in ALS remains ambivalent and unclear.