Giant parathyroid adenomas constitute a rare clinical entity, particularly in the developed world. We report the case of a 53-year-old woman where the initial ultrasonography significantly underestimated the size of the lesion. The subsequent size and weight of the adenoma (7cm diameter, 27g) combined with the severity of the hypercalcaemia raised the suspicion for the presence of a parathyroid carcinoma. This was later disproven by the surgical and histological findings. Giant parathyroid adenomas are encountered infrequently among patients with primary hyperparathyroidism, and appear to have distinct clinical and biochemical features related to specific genomic alterations. Cross-sectional imaging is mandated in the investigation of parathyroid adenomas presenting with severe hypercalcaemia as ultrasonography alone can underestimate their size and extent. This is important since it can impact on preoperative preparation and planning as well as the consent process as a thoracic approach may prove necessary for certain cases.