PURPOSE: This study sought to evaluate the histologic and mechanical properties of autologous in vivo tissue-engineered vascular grafts (in vivo TEVGs) used for pediatric heart surgery. DESCRIPTION: Molds of in vivo TEVGs made of silicone drain tubes were embedded into subcutaneous spaces in 2 boys during their first operation and were used as patch materials to treat pulmonary artery stenosis during the second operation. The remaining pieces of the patches were evaluated histologically and mechanically. EVALUATION: In vivo TEVGs had very smooth luminal surfaces, and their walls mainly comprised collagen fibers and small numbers of fibroblasts. Mean wall thickness was 200 μm, mean suture retention strength was 2.26 N, and burst pressure was 3057 mm Hg. CONCLUSIONS: Human in vivo TEVGs mainly comprise collagen fibers, and their mechanical properties prove them safe for pulmonary arterioplasty. Therefore, human in vivo TEVGs may be promising alternatives to autologous pericardium for pediatric cardiovascular surgical procedures that often require multistage operations.