Aim: To signify distinguishing features of atypical parathyroid adenoma and parathyroid carcinoma and throw light on immunohistochemistry as a new modality for diagnosis. Background: Solitary parathyroid adenomas (PAs) are the most common cause of primary hyperparathyroidism (PHPT). Generally, the PAs are small (<2 cm) and weigh approximately 70 mg to 1 g. Very rarely, some adenomas attain large sizes of >2 cm or weigh >2 g. These are termed as “Giant parathyroid adenoma (GPA).” Some of these may have few histological characteristics of parathyroid carcinoma (PC) but not sufficient to be diagnosed as cancer. These are categorized as atypical adenomas. We report a case of the largest atypical parathyroid adenoma. Case description: A 47-year-old gentleman, diagnosed with fracture shaft of the right femur, was found to have hypercalcemia (S. Cal 13.6) on routine biochemical examination along with deranged calcium metabolic profile and osteoporosis on bone scan. SestaMIBI scan and MRI reported a large PA in relation to right lobe of thyroid. Intraoperatively, a large adenoma measuring 6 × 4 × 3 cm was excised along with fracture fixation. Histopathology reported it as atypical parathyroid adenoma. Postoperatively, he developed hungry bone syndrome, which required intravenous and oral calcium and vitamin D supplementation. He became euglycemia 2 months after surgery. Conclusion: Atypical giant PA is a rare entity. Molecular phenotype of atypical adenomas are p27+, bcl+, ki67−, mdm+ but none of these is positive in carcinoma. Histopathology is a must for distinguishing these two, but this also may fail in tumors with equivocal features. Therefore, the paradigm for the diagnosis of these entities has shifted to immunohistochemistry, which holds a promising future. Clinical significance: Differentiating these two conditions is imperative in order to manage both and to keep patients of atypical parathyroid adenoma in close follow up.