We present a case of a greatly enlarged giant ectopic parathyroid adenoma that weighed 43 grams, which was located in the posterior mediastinum of a 74-year-old man. The patient presented with generalized weakness and decreased level of consciousness. He was found to have elevated level of serum calcium (19.9 mg/dl), and a subsequent assay of parathyroid hormone (PTH) was greatly elevated (2234 pg/ml). We report the course of management and outcome of the patient and present a review of the literature on giant ectopic parathyroid adenomas in the posterior mediastinum.