Objective: Parathyroid cancer (PC) represents < 1% of cases of PHPT. Tumors demonstrating atypical histopathologic features and don't fulfill criteria for carcinoma are classified as atypical adenomas (APA). The purpose of this study was to determine a biochemical or ultrasonographic feature that can predict aggressive disease requiring more extensive surgery and closer follow-up. Subjects and methods: Twenty eight patients operated for PHPT and diagnosed with atypical adenoma (23 patients) or carcinoma (5 patients) were enrolled in this study. The control group consisted of 102 patients operated between the same dates and diagnosed with classical PA. Classical adenomas, atypical adenomas, and carcinomas were compared according to their biochemical and ultrasonographic parameters. Results: Serum Ca levels were significantly higher in the PC group compared with the APA and classical PA groups. Serum median PTH, Serum ALP and UCa was significantly higher in the APA and carcinoma groups compared to the classical PA group. ROC analysis was made to determine the best cut off values for predicting aggressive disease were 12.45 mg/dL, 265.05 pg/mL, 154.5 IU/l, 348.5 mg/day and 21.5 mm for Ca, PTH, ALP, UCa and the adenoma diameter, respectively. Multivariate analysis showed that serum Ca, ALP and isoechoic/cystic appearance were independent predictors for aggressive disease. Conclusion: Preoperatively high PTH, ALP, and UCa levels and large lesions with isoechoic or cystic appearances may be predictive of atypical adenoma or carcinoma in patients being evaluated for PHPT. In such cases, surgeons may prefer en bloc parathyroidectomy to minimally invasive surgery.