Background Parathyroid carcinoma (PTCA) is an exceptionally rare malignancy, often with a clinical presentation similar to that of benign atypical parathyroid adenoma. Its low incidence portends unclear guidelines for management. Accordingly, thorough examination of clinical and pathologic variables was undertaken to distinguish between PTCA and atypical adenomas. Study Design This was a retrospective analysis of a prospective database at a tertiary academic referral center. Between September 2001 and April 2014, 3,643 patients were referred for surgical treatment of PHPT. Of these, 52 harbored aggressive parathyroid tumors: parathyroid carcinomas (n = 18) and atypical adenomas (n = 34). We analyzed the surgical and clinicopathologic tumor characteristics, and did a statistical analysis. We measured preoperative and intraoperative variables, and postoperative and pathologic outcomes. Results Parathyroid carcinoma patients present with significantly increased tumor size (3.5 cm vs 2.4 cm, respectively; p = 0.002), mean serum calcium (13.0 vs 11.8 mg/dL, respectively; p = 0.003) and intact parathyroid hormone (iPTH) levels (489 vs 266 pg/mL, respectively; p = 0.04), and a higher incidence of hypercalcemic crisis, compared with patients with atypical adenomas (50% vs 19%, respectively; p = 0.072). Parathyroid carcinoma more frequently lacks a distinct capsule (47.1% vs 12.9%, respectively; p = 0.03) and adheres to adjacent structures (77.8% vs 20.6%, respectively; p = 0.017). Of note, there was no significant difference in loss of parafibromin expression between groups. Conclusions Clinical distinction between PTCA and atypical adenomas is of critical importance in determining the appropriate extent of resection and follow-up. Loss of parafibromin has not been shown to distinguish between PTCA and atypical adenoma; clearer definition of clinicopathologic criteria for PTCA is warranted and may lead to improved postoperative management.