A 65-year-old male developed fatigable weakness of ocular and bulbar muscle and positive anti-acetyl cholinesterase antibodies suggesting the diagnosis of myasthenia gravis. His condition responded to anticholinesterase and immunotherapy. However, 18 months later, he developed painful paresthesiae, muscle cramps with hyperhiderosis, and was diagnosed as having Isaac's syndrome (neuromyotonia, continuous muscle fibre activity). Computed tomography of the chest revealed a thymic mass, which was confirmed after surgery and histopathology as thymic cell carcinoma. The co-occurrence of myasthenia gravis and continuous muscle fiber activity should prompt the consideration of the occurrence of these disorders as one of the paraneoplastic manifestations, most often due to a thymic neoplasm. Both these conditions respond to treatment of underlying thymoma. This case is a very rare presentation worth reporting.