Amongst the various methods that can be developed for noninvasive monitoring of gene expression in vivo, the use of positron emission tomography (PET) appears to be the most promising both for preclinical and clinical studies. Various genes have been described as potential PET reporters, but there is a need to develop new approaches that exploit transgenes with both therapeutic and imaging potential. The Na/I symporter (NIS) gene is expressed mainly in the thyroid and is responsible for iodide accumulation in this organ. The NIS gene has been used in gene therapy experimentation. Ectopic expression of this gene in various type of malignant cells has led to radiosensitization and in some cases tumor regression in xenograft models in nude mice, highlighting the therapeutic potential of this approach. In the present study, we demonstrate the potential of the human NIS gene (hNIS) as a reporter gene. Expression of hNIS, after plasmid transfection or adenoviral gene delivery, can be monitored in vitro on incubation with (125)I. Iodide uptake in the transduced cells can be directly correlated with the levels of gene expression in vitro. Ectopic expression of the NIS gene in vivo can be monitored in biodistribution studies on intravenous injection of (125)I. Adenovirus delivery induces gene expression essentially in the liver, adrenal glands, lungs, pancreas, and spleen. Expression of hNIS in tumor xenograft models can also be detected when the virus is injected intratumorally. Finally, hNIS expression was monitored by PET after intravenous injection of (124)I, demonstrating the potential of this approach for noninvasive imaging.