Patients with thymoma associated myasthenia gravis (MG) usually present with severe disease unresponsive to usual treatment and almost all have antibodies against acetylcholine receptors (AChR). We like to present a case of post-thymectomy, seronegative MG. A 55-year-old man was referred with horizontal diplopia and abduction deficit in the right eye two years after he had thymectomy for a thymoma. Single-fiber electromyography was diagnostic for MG and AChR titer was undetectable. Ocular signs did not response to pyridostigmine, prednisolone and immunoglobulin treatments and the patient also suffered from proximal weakness and pain in the extremities. Cyclosporine-A was started and the initial response was favorable with a decrease in general weakness.