Adult mesenchymal stem cells secrete a variety of angiogenic cytokines and growth factors, so we proposed that these paracrine mechanisms may be used to promote vascularization and growth for tissue engineering in vivo. We tested whether or not human adipose-derived stem cells (ASCs) promote tissue formation in rats. ASCs were evaluated in vitro for mRNA expression of angiogenic factors, including the vascular endothelial growth factor, basic fibroblast growth factor, interleukin-8 (IL-8), and stromal cell-derived factor-1 (SDF-1) and proliferative activity on human microvascular endothelial cells. For in vivo analysis, CM-DiI-labeled ASCs were implanted with a rat cardiac extracellular matrix (ECM) extract-derived hydrogel into a chamber with a femoral arteriovenous loop in the groin of male nude rats for 7 days. Vascularization in newly generated tissue was estimated by histomorphometry after endothelial nitric oxide synthase (eNOS) immunostaining. ASCs expressed growth factor mRNA and produced an angiogenic activity in vitro. After implantation, ASCs survived, but remained suspended in the ECM and relatively few were incorporated into the newly formed tissue. The volume of newly generated tissue was significantly higher in chambers containing ASCs and it was enriched with vasculature when compared with the ECM alone. We conclude that human ASCs promote tissue growth and angiogenesis in the rat vascularized chamber, thereby showing promise for tissue-engineering applications for regenerative therapy.