Thymoma occurs in about 20% of patients with myasthenia gravis (MG), but the recurrence of thymoma has rarely been reported. A 68-year-old man presented with ptosis and weakness of the neck muscles. He had undergone thymectomy for ocular MG with invasive thymoma 20 years previously. Chest computed tomography revealed recurrence of the thymoma. This 20-year period is, to our knowledge, the longest reported interval for recurrence of an MG-associated thymoma. The literature on the optimal follow-up duration after thymectomy is reviewed.