Adrenomedullin (ADM) is a new potent vasorelaxant peptide identified originally in extracts of pheochromocytoma, and is widely distributed within the tissue. Although histopathological studies have demonstrated the presence of ADM-immunoreactivity (ir-ADM) in some human neuroendocrine tumors (such as insulinoma, pituitary adenoma, and gastrointestinal neuroendocrine tumors), data on the presence of ADM in normal and pathological parathyroid gland are not available. Plasma AM concentrations were recently reported to be elevated in patients with PHP (primary hyperparathyroidism). The aim of our study was to determine tissue distribution of ir-AM in 34 patients with PHP (27 female and 7 male, mean age 50 +/- 6 years) undergoing surgery. Six normal parathyroid samples incidentally found during thyroidectomy for neoplastic diseases and ten sections of human rectus abdominis muscle tissue were used as controls (C). Adenomatous parathyroids were found in 22 PHP and hyperplastic parathyroids in twelve PHP patients. Four hyperplastic parathyroids were found in three PHP patients and three parathyroids in 10 PHP patients. Eight parathyroids revealed a prevalent diffuse growth pattern and four showed a prevalent nodular growth pattern. Immunohistochemical ADM expression was seen in seven of twelve (58.3 %) hyperplastic parathyroids and in fourteen of twenty-two (66.6 %) adenomatous glands. Parathyroid chief cells showed strong cytoplasmatic staining, whereas oncocytic cells showed a faintly aspecific cytoplasmatic staining. Normal parathyroids were negative for ir-ADM. In conclusion, we found the presence of ADM in parathyroid chief cells of PHP patients using immunohistochemistry in our study.