[en] This study was designed to evaluate the ultrasonographic (US) findings of medullary thyroid carcinoma (MTC) as compared to findings for papillary thyroid carcinoma (PTC). The study included 21 cases of MTC that were surgically diagnosed between 2002 and 2007 and 114 cases of PTC that were diagnosed in 2007. Two radiologists reached a consensus in the evaluation of the US findings. The US findings were classified as recommended by the Thyroid Study Group of the Korean Society of Neuroradiology and Head and Neck Radiology (KSNHNR) and each nodule was identified as suspicious malignant, indeterminate or probably benign. The findings of medullary and papillary carcinomas were compared with use of the chi-squared test. The common US findings for MTCs were solid internal content (91%), an ovoid to round shape (57%), marked hypoechogenicity (52%) and calcifications (52%). Among the 21 cases of MTC nodules, 17 (81%) were classified as suspicious malignant nodules. The mean size (longest diameter) of MTC nodules was 19 ±13.9 mm and the mean size (longest diameter) of PTC nodules was 11 ± 7.4 mm; this difference was statistically significant (p < 0.05). An ovoid to round shape was more prevalent for MTC lesions than for PTC lesions (p < 0.05). The US criteria for suspicious malignant nodules as recommended by the Thyroid Study Group of the KSNHNR correspond to most MTC cases. The US findings for MTC are not greatly different from PTC except for the prevalence of an ovoid to round shape