We report a case of a 35-year-old male, with a history of diarrhea, renal lithiasis with frequent expulsions of calculus and hypercalcemia during the last 2 years. The patient was studied and diagnosed with a multiple endocrine neoplasia type I (MEN I), familiar (mother with MEN I). A scintigraphic study with 99mTc-MIBI was performed in order to localize hyperfunctioning parathyroid glands because of biochemical diagnosis of primary hyperparathyroidism. Double phase 99mTc-MIBI scan detected one hyperfunctioning parathyroid gland and a large anterior mediastinal mass. Subsequent, plain radiograph and CT of the chest showed a soft-tissue mass in that localization. Punch biopsy of the lesion guided by CT revealed malignant cells of neuroendocrine tumor. The tumor was removed and histologically confirmed as a carcinoid within a thymus in a MEN type I syndrome. MEN I patients can benefit from the examination with this agent which can potentially localize not only parathyroid endocrine pathology but also unknown associated tumors.