Background & Aims: The ability of thyroid cells to take up iodide, which enables I-131 radiotherapy for thyroid cancer, is due to the expression of the sodium iodide symporter at their plasma membrane. Expression of this symporter has been found in some non-thyroid cancers. However, it is mostly accumulated in the cytoplasm, and its functionality has not been demonstrated. We have investigated sodium iodide symporter expression and functionality in human liver cancer, and in a diethylnitrosamine induced Wistar rat model of primary liver cancer at different stages of carcinogenesis. Methods: Sodium iodide symporter mRNA and protein were studied in tissues from patients with hepatocellular- or cholangiocarcinomas using reverse-transcription polymerase chain reaction, immunoblot, and immunohistochemistry. We studied the dynamics of hepatic iodine uptake in the animal model using nuclear imaging. Results: Sodium iodide symporter expression showed up in all 20 cholangiocarcinomas, but in only 2 of the 26 hepatocellular carcinomas, investigated. It was also found in normal bile duct cells and in the ductular reaction present in cirrhotic tissues. It was located at the plasma membrane in 10 of 20 cholangiocarcinoma. In rat liver cancer, a functional sodium iodide symporter expression was triggered as from the early preneoplastic steps, and was amplified during clonal tumor cell expansion, allowing complete tumor suppression after I-131 radiotherapy. Conclusions: A significant proportion of human cholangiocarcinomas expresses membrane sodium iodide symporter, which may permit radioiodine therapy. Our data also suggest that I-131 acts on a crucial target for liver cancer development.