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Case summary A 4-month-old cat had bilateral swellings of the mandible, maxilla, humerus and femur, and angular deviations in the axial and appendicular skeleton. The biochemical profile indicated hypercalcemia, hyperphosphatemia and increased parathyroid hormone levels. Because of the poor prognosis, the cat was euthanized. At necropsy, malleable and fragile bones, associated with numerous cystic areas containing yellowish and translucent liquid, were observed. Histologically, the bones showed marked diffuse proliferation of fibrous connective tissue, and large numbers of osteoclasts surrounding numerous cystic structures were also observed within fibrotic areas at the periphery of the trabecular bone. In addition, enlargement of the parathyroid glands, which was associated with increased serum concentrations of calcium, phosphorus and parathyroid hormone, was detected. Relevance and novel information The changes observed in this cat are consistent with hyperparathyroidism-associated osteitis fibrosa cystica, which is an unusual presentation in the cat. Hyperparathyroidism, either primary (neoplastic) or secondary (nutritional or renal), is the primary cause of this condition.