This report describes light and ultrastructural features of a functional parathyroid gland adenoma, principally composed of transitional oxyphil cells, in a 64-yr-old hypertensive black woman. She was hospitalized for repeated episodes of headaches, lethargy, and dizzy spells. Her serum calcium level was 2.92 mmol/l and immunoassay for parathormone was 390 pg/ml. On neck exploration, the left lower parathyroid gland was found enlarged and therefore removed in toto. The serum calcium and phosphate levels returned to normal following parathyroidectomy. Microscopically, the diagnosis of functional oxyphil adenoma was made. On ultrastructural examination, the tumour was composed principally of transitional cells, occasional typical, and degenerating oxyphil cells. The predominant transitional cells were rich in mitochondria and contained multiple active Golgi complexes, stacked profiles of rough endoplasmic reticulum, and a few secretory granules. On the other hand, typical oxyphil cells were tightly packed with mitochondria at the expense of other organelles. It appeared that neoplastic oxyphil cells were chief eel Is transformed in response to some unknown oncogenic stimulus.