PURPOSE: To evaluate US, RI and CT findings of coexistent thyroid cancer with Hashimoto's thyroiditis. MATERIALS AND METHODS: In 104 patients with Hashimoto's thyroiditis, 27 patients with coexistent cancer were reviewed. The assessment of US, RI and CT was done. RESULTS: Twenty-four out of 27 cases were papillary (89%), 2 follicular (7%) and 1 papillary adenocarcinoma with lymphoma (4%). Sonographically, dominant finding of Hashimotos thyroiditis was heterogeneous (96%) low echoic (79%) parenchyme and most nodules were hypoechoic (88%). Calcifications were observed in 3.2 (50%) malignancies. RI scan showed varying degree of thyroid enlargement in 18 patient (67%), and 15 cold (54%) and 2 hot (7%) nodules. CT findings were as follows.: thyroidal enlargement (79%), heterogeneous enhancement (79%), nodular low density (36%), calcification (21%), cervical lymphadenopathy (21%) and muscle invasion (7%). CONCLUSION: The sonographic detection of hypoechoic nodule with calcification in Hashimoto's thyroiditis can be a clue of coexistent cancer.