A patient who was evaluated for a voice change was found to have a large posterior mediastinal mass on chest roentgenogram. Laboratory parameters suggested hyperparathyroidism. The 190-g resected tumor had the histologic features of a parathyroid lipoadenoma, that is, a diffuse mixture of parathyroid glandular elements and fat or myxoid stroma throughout. This case report emphasizes that the large size of these rare tumors may result in herniation into unusual sites, and this gives rise to a confusing clinical presentation. Ultrastructural analysis showed a uniform population of chief cells with a distribution of cellular organelles consistent with slow hormone production and release. Similarities to normal and adenomatous chief cells are discussed. The myxoid stroma surrounding the glandular elements had the ultrastructural features of loose connective tissue.