Vascular adventitia is thought to change functions and contribute to diseases such as atherosclerosis, vascular restenosis, and fibrosis. To determine whether the adventitia contains mesenchymal stem/progenitor cells (MPCs), we cultured human vascular adventitial fibroblasts (hVAFs) from pulmonary arteries and analyzed their characteristics. The doubling time of the hVAFs was 1.5 days, and the average number of passage was 11, which was independent of age and sex. The hVAFs were positive for vimentin, collagen type-1, CD29, CD44, and CD105, but negative for hematopoietic and endothelial cell markers. When hVAFs were cultured in appropriate media, they showed osteogenic and adipogenic differentiation by von Kossa, alkaline phosphatase, and oil red 0 staining. Myogenic differentiation was identified, by increased expression of smooth muscle actin and calponin. These findings demonstrate that human vascular adventitia contains MPCs, and that hVAFs may be an ideal source for further experiments on stem cell biology and tissue engineering. (C) 2008 Elsevier Inc. All rights reserved.