The association between RBD and synucleinopathies is well known. However, the association between RBD and other neuromuscular diseases has not been as well described. Our case study describes two siblings with familial ALS, confirmed by the identification of the L84F mutation in the SOD1 gene, and RDB. We hope this case study will promote future studies on the prevalence of this association and will stimulate research in identifying the underlying pathogenic mechanism.