To monitor noninvasively potentially therapeutic adenoviruses for cancer, we have developed a methodology based on the sodium iodide symporter (NIS). Men with clinically localized prostate cancer were administered an intraprostatic injection of a replication-competent adenovirus, Ad5-yCD/utTK(SR39)rep-hNIS, armed with two suicide genes and the NIS gene. NIS gene expression (GE) was imaged noninvasively by uptake of Na(99 m)TcO(4) in infected cells using single photon emission-computed tomography (SPECT). The investigational therapy was safe with 98% of the adverse events being grade 1 or 2. GE was detected in the prostate in seven of nine (78%) patients at 1 x 10(12) virus particles (vp) but not at 1 x 10(11) vp. Volume and total amount of GE was quantified by SPECT. Following injection of 1 x 10(12) vp in 1 cm(3), GE volume (GEV) increased to a mean of 6.6 cm(3), representing, on average, 18% of the total prostate volume. GEV and intensity peaked 1-2 days after the adenovirus injection and was detectable in the prostate up to 7 days. Whole-body imaging demonstrated intraprostatic gene expression, and there was no evidence of extraprostatic dissemination of the adenovirus by SPECT imaging. The results demonstrate that noninvasive imaging of adenovirus-mediated gene therapy in humans is feasible and safe.