Primary hyperparathyroidism was identified in a 17-year-old Arab x Welsh Pony mare that experienced weight loss for 6 months and was presented with mild facial asymmetry, right forelimb lameness and weight shifting amongst all limbs. Osteodystrophia fibrosa was demonstrated on radiographic examination of the head and there was radiographic evidence of osteopenia of the appendicular skeleton. The horse had persistent hypercalcaemia (4.0 mmol/l), hypophosphataemia (0.59 mmol/l) and an increased concentration of circulating parathyroid hormone (1401 pg/ml). On scintigraphic examination, a subjective focal increase in uptake of 99mtechnetium-sestamibi was identified in the right thyroid gland and at the thoracic inlet in delayed images. Surgical exploration of the thyroid region was unrewarding, whereas surgery at the thoracic inlet was not undertaken. One year later, the horse developed a pathological fracture of the third metacarpal bone and was subjected to euthanasia. At post mortem examination, a parathyroid adenoma was found at the level of the thoracic inlet adjacent to the trachea. Gross and histological examination also confirmed severe osteodystrophia fibrosa and osteopenia.