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A 36 year old Portuguese man was admitted to our hospital with diffuse malaise, 25 kg weight loss in the past 6 months and night sweats. The diagnostic workup revealed hyperthyroidism due to graves disease and a mass lesion in the upper anterior mediastinum. Because of the uncertain dignity of the tumor a thoracoscopic thymectomy was performed. Histologically we found a thymic hyperplasia without any signs of malignancy. If systematically searched, thymic hyperplasia is often found with graves disease, however, this association is not widely appreciated and is not mentioned in general text books. The pathophysiology of thymic hyperplasia in graves disease is not fully understood. Immunological mechanisms seem to play an important role, since TSH receptors have been found on human thymic epithelial cells. Thyreostatic therapy leeds to spontaneous shrinkage of the thymic gland. Awareness of thymic hyperplasia in the differential diagnosis of an anterior mediastinal mass lesion is essential, since it prevents further invasive diagnostic procedures that might cause unnecessary morbidity.