Morvan's syndrome is a rare disease characterized by peripheral nerve hyperexcitability, associated with CNS and autonomic systems involvement. High serum voltage-gated potassium channel (VGKC) antibody titers have been reported, and, till now, Morvan's syndrome has been considered as a VGKC antibody associated disease. We describe a patient with Morvan's syndrome associated with myasthenia gravis and a thymoma in his previous history, with surprisingly undetectable levels of VGKC antibodies. The clinical course is similar to those cases of Morvan's syndrome with VGKC-Ab, except for the lack of response to plasma exchange, previously considered as the first choice treatment. Nevertheless, the good response to corticosteroids therapy and the association with myasthenia confirm an autoimmune origin of the disease.