Myasthenia gravis is a chronic autoimmune disease characterized by weakening of voluntary muscles during the day and a marked restitution of function during the night and after rest. The symptoms may worsen over days or weeks, sometimes even in a few hours, and are usually well controlled by appropriate therapy. Arachnoid cysts are congenital or acquired deformities of the arachnoid membrane and are usually too small to cause distinct clinical symptomatology. We describe a case of a 76-year-old myasthenia gravis patient with an arachnoid cyst. To the best of our knowledge this is the first reported case of these two comorbidities together.