Adequate antithyroid drug treatment or surgery usually generates remission of myasthenia gravis (MG) in patients with thymus hyperplasia associated with Graves' hyperthyroidism. The case of a 46-year-old woman diagnosed with MG based on the clinical picture, anticholinesterase drug test and positive electromyography (EMG) is presented. The cervico-thoracic computer tomography revealed a compressive nodular goiter and normal antero-superior mediastinum and led to the diagnosis of MG secondary to the hyperthyroidism. An uneventful total thyroidectomy was performed, but postoperatively the MG symptoms worsened. TC99m tetrofosmin scintigraphy revealed an area of hyperfixation in the antero-inferior mediastinum, suggestive for thymoma, as confirmed by a repeated thoracic CT scan. Following a longitudinal sternotomy, a well incapsulated tumor of approximately 6/5 cm located in the antero-inferior mediastinum was found and an extensive thymomectomy was performed. The postoperative course was uneventful and the patient was discharged 9 days later with complete remission of myasthenia. The pathology report of the specimen revealed a mixt thymoma or AB thymoma after Muller-Hermelink and WHO classification, with invasive capsular foci corresponding to Masaoka II stadium. In conclusion, scintigraphy proved to be useful in the diagnosis and decision making of a thymoma.