A 1-year-old, 267 kg Morgan filly presented with progressive unilatera
l facial swelling. Physical, radiographic, and gross pathological find
ings suggested a diagnosis of maxillary and conchal cystic osteodystro
phia fibrosa which was verified by microscopic examination. Pronounced
hypertrophy of the parathyroid glands supported the etiologic diagnos
is of nutritional secondary hyperparathyroidism.