In human cells, telomerase activity is regulated by transcriptional control of the telomerase reverse transcriptase gene (hTERT) whose product is the catalytic subunit of the enzyme. The hTERT promoter is active in virtually all types of tumors and immortal cells, but is silent in most adult somatic tissues. In this study, we placed the herpes simplex virus thymidine kinase gene under the control of the hTERT promoter with the aim of restricting its expression to tumor cells. In transfection experiments, the hTERT promoter driven thymidine kinase gene (hTERTp/TK) conferred ganciclovir sensitivity to all tumor and immortal cell lines tested, whereas normal somatic cells remained largely unaffected. Human hTERTp/TK-positive cancer cells implanted in nude mice developed into tumors that could be eradicated by ganciclovir treatment. The hTERTp/TK cassette was inserted into an adenovirus vector and its efficacy in reducing tumor growth was compared with that of an adenovirus carrying the thymidine kinase gene under the control of the cytomegalovirus immediate-early promoter (CMVp/TK). In a xenograft model using the human 143B osteosarcoma cell line, a single injection of either virus resulted in equivalent tumor regression and survival upon ganciclovir treatment. In animals injected intratumorally with the CMVp/TK adenovirus, expression of the thymidine kinase gene was detected in tumors, as well as in liver samples. Expression of the suicide gene in combination with ganciclovir resulted in severe liver histopathology and in an elevation of hepatic enzymes. In sharp contrast, when the hTERT promoter controlled the thymidine kinase gene, transgene expression was observed in tumors, but not in liver samples. Normal liver function in these animals was confirmed by serum levels of hepatic enzymes that were indistinguishable from those of control healthy mice. These results indicate that by restricting thymidine kinase expression to tumor cells, the hTERT promoter allows the tumoricidal effect of the suicidal gene to be exerted without detrimental consequences on healthy tissues and vital organs. The tight specificity of expression imparted by the hTERT promoter will assist the development of novel approaches to the treatment of a broad array of cancer types.