Case summary A 16-year-old neutered female Korat cat presented with chronic vomiting, mild azotaemia and mild hypercalcaemia. Physical examination revealed bilateral palpable masses on each side of the trachea. Laboratory results were consistent with primary hyperparathyroidism, diagnostic imaging findings with cystic thyroid or parathyroid masses, and fine-needle aspiration cytology with thyroid hyperplasia or adenoma. In order to confirm whether one or two of the masses were the cause of the hyperparathyroidism, cystic fluid was aspirated from both for parathyroid hormone concentration measurement. The concentration was shown to exceed that of the serum manyfold in both samples, confirming both masses to be functional and of parathyroid origin. A total parathyroidectomy and thyroidectomy were performed on the right side, and a subtotal thyroidectomy and a subtotal to total parathyroidectomy on the left, without any major postoperative complications. Histopathology was consistent with bilateral parathyroid carcinoma. Relevance and novel information To our knowledge, this report is the first to describe a rare case of bilateral parathyroid cystic carcinoma in a cat. It highlights the usefulness of determining parathyroid hormone concentration in the cystic fluid of a suspected neoplastic parathyroid mass preoperatively. It also demonstrates that it may be possible to remove most of the cervical parathyroid and thyroid tissue of a cat without causing any clinically relevant hypocalcaemia or iatrogenic hypothyroidism. However, serum concentrations of ionised calcium, thyroxine and creatinine should be closely monitored in the postoperative period in order to detect and control possible complications.