PURPOSE: The purpose of this study was to categorize macrocalcifications into several subtypes by the US findings and to determine which type of macrocalcification in a thyroid nodule is associated with thyroid malignancy. MATERIALS AND METHODS: We retrospectively analyzed the macrocalcification patterns of thyroid nodules in 396 patients that underwent ultrasonography (US)-guided thyroid FNA or surgery in our institution between August 2009 and August 2011. Two radiologists evaluated US findings and categorized macrocalcifications into 5 subtyes : (A) solitary macrocalcification no association with thyroid nodule; (B) nodular macrocalcification(s) within indeterminate thyroid nodule; (C) dense macrocalcification without ability to interpret internal content by its posterior acoustic shadowing; (D) irregular-shaped macrocalcification(s); (E) macrocalcification with other suspicious malignant US finding(s). A chi-squared test and a Fisher exact test were used for comparison of categoric variables. The diagnostic sensitivity, specificity, positive and negative predictive values (PPV and NPV) were obtained. RESULTS: Among total of 417 nodules, 114 (27.3%) were suspicious malignancy or malignancy on histopathological result. Macrocalcification with other malignant US feature had the highest incidence of malignancy (77.5%), followed by irregular-shaped macrocalcification(s) (43.9%), and dense macrocalcification without ability to interpret internal content by its posterior acoustic shadowing (38.5%), solitary macrocalcification no association with thyroid nodule (8.3%) respectively, and nodular macrocalcification(s) with indeterminate thyroid nodule had the lowest incidence (7.5%). A nodule with macrocalcification with other malignant US finding(s) has the highest odds ratio (42.52), followed by a nodule with irregular-shaped macrocalcification(s) (9.65) and dense macrocalcification (7.72). Leaving macrocalcification with compositive malignant findings aside, irregular-shaped macrocalcification is more likely to be associated with increased risk for malignancy compared with other patterns of macrocalcification. CONCLUSIONS: Irregular-shaped macrocalcification is a fine indicator for differentiation between benign and malignant thyroid nodules on ultrasonography.