In this report, a case of differentiated thyroid carcinoma having false-positive I-131 accumulation due to a large hydatid cyst in the liver is presented. Abdominal sonographic examination of a 37-year-old patient with differentiated thyroid carcinoma demonstrated an ovoid, complex cystic mass containing cystic parts as well as solid-appearing components in the right lobe of liver, which is concordant with a type CE-3 hydatid cyst. The postablative whole-body scan obtained 10 days after 100 mCi I-131 therapy displayed intense and homogeneous uptake of radioiodine in the liver. Hepatic cystic mass was removed, and postoperative histologic evaluation confirmed hydatid disease. Some cystic lesions of the kidney and liver have been previously reported as causes of false-positive radioiodine accumulation.