Ectopic tumoral secretion of authentic PTH is rare, as only four cases have been convincingly documented by demonstrating the presence of PTH messenger ribonucleic acid in tumor tissue. We report the case of a 25-yr-old male with biochemical alterations typical of primary hyperparathyroidism (elevated calcemia and renal tubular reabsorption of calcium, decreased phosphatemia and maximal tubular reabsorption of phosphate, and increased intact PTH serum levels). Extensive cervical exploration did not reveal any abnormally enlarged parathyroid tissue, but excision of a palpable superior retrosternal mass led to the correction of all abnormal biochemical values. Histological analysis showed a predominantly epithelial thymoma, without any detectable parathyroid gland on serial slices. Tumor extracts contained immunoreactive PTH material, with serial dilutions paralleling PTH standards in an immunoradiometric assay. By contrast, immunoreactive PTH-related protein was absent. Furthermore, on Northern blot analysis, there was a PTH messenger ribonucleic acid transcript with a size similar to that found in parathyroid adenoma or hyperplasia. The thymoma epithelial cells stained positively with antiserum against PTH-(1-34), but negatively with antichromogranin-A antiserum. These results support the ectopic production of authentic PTH by a thymoma and indicate a novel tumoral cause of primary hyperparathyroidism.