Giant clear cell parathyroid adenomas (GPA) are rare parathyroid tumours (1). Importantly, they can mimic parathyroid carcinoma, which is also rare and accounts for less than 1% of all primary hyperparathyroidism cases (2). Differentiating between the two preoperatively is difficult, as both are associated with large tumour size, high calcium and PTH levels. (1) We present a case of a GPA in a 60 year old female with primary hyperparathyroidism. Preoperative imaging showed a 6.5cm lesion posterior to the right thyroid lobe concerning for parathyroid carcinoma. The patient was consented for parathyroidectomy +/- en bloc resection of thyroid and recurrent laryngeal nerve. Intraoperatively the lesion was assessed by two endocrine surgeons and appeared clinically benign. The decision was made to proceed with parathyroidectomy only. Histology confirmed the lesion to be a GPA of water clear type. This case adds to the literature on a rare disease. It highlights the difficulty with preoperative diagnosis and importance of intraoperative assessment with regards to differentiating parathyroid carcinoma from benign GPA. References 1.Wong HK, Shipman K, Allan K, et al. Giant parathyroid tumours in primary hyperparathyroidism: a systematic review. Langenbecks Arch. Surg. 2022 Jan;407:501-506 2.Ruda J, Hollenbeak CS, Stack BC. A systematic review of the diagnosis and treatment of primary hyperparathyroidism from 1995-2003. Otolaryngol. Head Neck Surg. 2005 March;132:359-372