We developed a panel of lentiviral vectors that displayed tetracycline-regulated transgene expression over two orders of magnitude in bulk, non-selected populations of transduced cells in vitro and in vivo. The robust expression and homogeneous response indicated that most transduced vector genomes were transcription competent and responsive to regulation, providing the lentiviral vector with a novel competitive advantage for gene transfer. After ex vivo transduction and transplantation of cord blood CD34+ cells into NOD/SCID mice, reporter gene expression could be switched "on" and "off" in human hematopoietic cells in vivo for prolonged times, proving integration of the regulated expression system into long-term repopulating cells. By vector injection into established tumor grafts, we achieved efficient delivery and quantitative regulation of transgene expression in vivo. By these approaches, gene function studies can now be performed in in vivo models of human hematopoiesis and cancer. In the future, regulated lentiviral vectors will improve the safety and efficacy of gene therapy.