In vivo transduction of the herpes simplex virus type-1 thymidine kinase (HSV-1 TK) gene and subsequent administration of antiviral drugs such as ganciclovir has emerged as a promising gene therapy protocol for proliferative disorders. However, the evaluation of HSV-1 TK gene expression in transduced tissue has relied on invasive techniques for detection. We now report that HSV-1 TK expression can be detected non-invasively using scintigraphy. The radioiodinated nucleoside analogue, (E)-5-(2-iodovinyl)-2'-fluoro-2'-deoxyuridine (IVFRU), becomes metabolically trapped in tumour cells transduced with the HSV-1 TK gene on a retroviral vector. Selective phosphorylation of radiolabelled IVFRU by HSV-1 TK results in elevated radioactivity in HSV-1 TK-expressing cells in vitro and in vivo relative to cells lacking the HSV-1 TK gene. Due to low non-target tissue uptake, unambiguous imaging of HSV-1 TK-expressing tumours in mice is possible with labelled IVFRU. We have monitored the process of tumour regression non-invasively during ganciclovir treatment using labelled IVFRU and scintigraphy.