Although several authors have reported single cases illustrative of some ultrasonographic characteristic of parathyroid carcinoma, the value of ultrasonography for diagnosing this entity remains to be determined. The purpose of our study was to investigate the ultrasonographic features of parathyroid carcinoma in a large number of cases. We assessed the shape, contour, echogenicity, and depth-width (DW) ratio of 16 parathyroid carcinomas and 61 parathyroid adenomas. Ultrasonography showed that parathyroid carcinomas tend to be large, inhomogeneous, hypoechoic masses with lobulated contours. In contrast, parathyroid adenomas were small, homogeneous, hypoechoic masses with smooth borders. The mean (range) DW ratios for parathyroid carcinomas were 1.21 (0.91-2.5) and 0.64 (0.33-1.47) for adenomas; the difference was statistically significant (p<0.0001). The DW ratio was ≥1 in 15 (94%) of the 16 cases of carcinoma, whereas only 3(5%) of the 61 adenomas had a similar ratio. Ultrasonographic examination is useful not only for preoperative localization but also for differentiating parathyroid carcinoma from adenoma. Parathyroid tumors with irregular margins, inhomogeneous echogenicity, and a DW ratio ≥1 are likely to be malignant.