Peripheral giant cell granulomas (PGCG) associated with hyperparathyroidism (HPT) are rare clinical entities. The aim of this study is to report on 21 PGCGs of the oral cavity as the first clinical sign of unknown primary HPT (PHPT) referred to the Complex Operating Unit of Odontostomatology of Aldo Moro University of Bari from 2009 to 2019. Surgical treatment consisted in conservative enucleation of the lesion, if possible, with contextual bone rim osteoplasty with piezosurgical tools and following histological examination. After histological diagnosis of PGCG, PHPT screening was performed dosing parathyroid hormone and serum calcium. In all the patients haematological investigation demonstrated elevated values of parathyroid hormone and serum calcium ruling out an unknown PHPT. Specifically, after endocrinological evaluation, patients showed PHPT related to: parathyroid adenoma (13), parathyroid hyperplasia (two, one of which occurred in a intra-thyroidal parathyroid), and parathyroid carcinoma (1) and were scheduled for surgical treatment. Considering that PGCGs could represent the first clinical sign of an undiagnosed PHPT and the screening of PHPT is a non-invasive and cheap exam, in case of histological diagnosis of a giant cell lesion, both central and peripheral, especially in patients with synchronous or history of methacronous giant cell lesions, parathyroidal screening should be mandatory.