Parathyroid cysts located in the mediastinum are rare. They may be non-functioning or associated with primary hyperparathyroidism (PHPT). We present a patient with persistent PHPT despite previous parathyroid surgery. Parathyroid scintigraphy with 99mTc sestamibi showed no focus with radioactivity retention, but MRI revealed a large parathyroid cystadenoma in the mediastinum, which was successfully removed. In general, MRI is not the first choice for parathyroid imaging, but when the adenoma is localized at ectopic sites, MRI is a good imaging modality.