Purpose <p>Parathyroid carcinoma is an extremely rare malignancy, with an unfavorable clinical outcome and a high recurrence rate after initial surgery. Because surgical removal without tumor cells is the only curative treatment, a preoperative diagnosis is important to develop a strategy for complete removal of tumor cells. This study aimed to examine preoperative methods to differentiate parathyroid carcinoma from parathyroid adenoma.</p> Methods <p>We retrospectively analyzed the clinical and pathological features of patients with parathyroid carcinoma or parathyroid adenoma who were undergoing treatment at our hospital between 2011 and 2021.</p> Results <p>The statistical analysis included 7 patients with parathyroid carcinoma and 132 patients with parathyroid adenoma. For the preoperative diagnosis of parathyroid carcinoma, we found that the serum intact parathyroid hormone (iPTH) level was the most useful indicator, showing a significant positive correlation with depth to width (D/W) ratio (<i>r</i>^<i>2</i> = 0.6084, <i>p</i> = 0.0386) in parathyroid carcinoma, whereas no correlation was observed in parathyroid adenoma. Furthermore, in patients with a D/W ratio of &gt; 0.534, iPTH levels more clearly differentiated parathyroid carcinoma from parathyroid adenoma.</p> Conclusion <p>The combination of the serum iPTH level and D/W ratio may be useful in the differential diagnosis of parathyroid carcinoma.</p>