Here we report the analysis of the Na(+)/I(-) symporter (NIS) protein expression in 57 thyroid cancer samples by immunohistochemistry with high-affinity anti-NIS Abs. As many as 70% of these samples exhibited increased NIS expression with respect to the normal surrounding thyroid tissue. Most significantly, NIS was located in these samples either in both the plasma membrane and intracellular compartments simultaneously, or exclusively in intracellular compartments. This suggests that NIS is clearly expressed or even overexpressed in most thyroid cancer cells, but malignant transformation in some of these cells interferes either with the proper targeting of NIS to the plasma membrane, or with the mechanisms that retain NIS in the plasma membrane after it has been targeted. The results further indicate that, in addition to indicating NIS expression in cases where it is absent (approximately 30%), improvements in (131)I radioablation therapy might result from promoting targeting of NIS to the plasma membrane in the majority (approximately 70%) of thyroid cancers.