Noninvasive imaging of herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression is possible with a clinical gamma camera and by single-photon emission tomography (SPECT) using ¹³¹I-labeled 2′-fluoro-2′-deoxy-1-β-d-arabinofuranosyl-5-iodo-uracil (FIAU). Studies were performed in rats bearing s.c. tumors. Tumors were produced by injection of wild-type RG2 glioma or W256 mammary carcinoma cells into one flank and RG2TK+ glioma or W256TK+ mammary carcinoma cells (that had been transduced in vitro with the HSV1-tk gene) into the opposite flank. In some animals, HSV1-tk gene transduction of the pre-established wild-type tumors was accomplished in vivo by direct intratumoral injection of retroviral vector-producer cells. Imaging studies were performed 2 weeks after tumor transduction to allow time for production and spread of the retroviruses through the tumor and for sufficient growth and increase in size of the tumors to facilitate imaging. The gamma camera and SPECT images revealed highly specific localization of [¹³¹I]FIAU-derived radioactivity to areas of HSV1-tk gene expression at 24, 36, and 48 h after i.v. administration of 1.6–2.8 mCi of [¹³¹I]FIAU. Comparative analysis of quantitative autoradiographic images obtained from the same tumors confirmed that the high levels of [¹³¹I]FIAU-derived radioactivity (>1% dose) were localized to areas of HSV1-tk gene expression demonstrated by immunohistochemical staining for HSV1-tk protein. In contrast, significantly lower levels of [¹³¹I]FIAU-derived radioactivity (<0.01%) were observed in the surrounding nontransduced tumor tissue, contralateral wild-type tumors, and other tissues that showed no immunohistochemical staining for the HSV1-tk protein. The magnitude of FIAU accumulation in RG2TK+, W256TK+, and wild-type tumors corresponded to the in vitro ganciclovir sensitivity of the cell lines used to produce these tumors, which indicates that the magnitude of FIAU accumulation reflects the level of HSV1-tk gene expression. We suggest that “clinically relevant” levels of HSV1-tk gene expression in transfected tissue can be imaged with [¹³¹I]FIAU and a gamma camera or SPECT, and that a significant improvement in imaging sensitivity and resolution is expected with [¹²⁴I]FIAU and PET.