Studies of animals with spontaneous autoimmune diabetes have revealed that autoreactive T cells that mediate islet β cell destruction can be manipulated by the administration of Th2 cytokines. Using gene delivery to express the targeted protein, we can overcome the need for frequent administration of cytokines on account of their short half-lives. In this study, the effect of hTGFβ gene delivery was evaluated both in vitro and in vivo using an adenovirus vector (Ad) constructed with an hTGFβ cDNA. In vitro transfection assays of the construct in HepG2, β cell lines, and islets showed good expression levels of hTGFβ and activation of smad3. Ad-hTGFβ enhanced differentiation and proliferation in the β cell line or islets without causing apoptosis. Of interest, Ad-hTGFβ transduction in CD4+CD25- T cells resulted in a significant enhanced expression of CD25 and a regulatory T cell-specific transcription factor, Foxp3. To evaluate in vivo efficacy, Ad-hTGFβ was intravenously injected into 7-week-old NOD mice and compared to the transduction using the vector only. The Ad-hTGFβ group had persistent gene expression for longer than 5 weeks, and high TGFβ serum level was secreted. There was no difference in the degree of insulitis between the Ad-hTGFβ group and controls. Although we found favorable in vitro results, such as decrease in islet apoptosis, enhanced proliferation and differentiation, and increase in the level of CD4 +CD25+ regulatory T cells, there was no difference in reduction of the development of T1D between controls and Ad-hTGFβ-injected mice. Nevertheless, if we find the appropriate mode and timing of TGFβ gene transduction, Ad-hTGFβ gene therapy might be useful in therapeutic cytokine delivery for the treatment of T1D.