Objectives: Our previous study demonstrated the potential for iPS cells to be used as a new cell source for tracheal regeneration therapy (Imaizumi, et al, 2010). However, teratoma (tumor) formation remains a major problem limiting the use of iPS cells. Cell line- and implantation site- dependent differences in teratoma formation have been reported (Miura, et al, 2009). In the current study, the teratomaforming propensity after implantation into tracheal defects and abdominal subcutaneous tissue was examined histologically and quantitatively. Methods: Mouse iPS cells were cultured in artificial material under various conditions. After cultivation in vitro, each of iPS cells was examined histologically. Artificial materials with cultured iPS cells were then implanted into the tracheal defects and into the abdominal subcutaneous tissue in nude rats. Teratoma formation was evaluated histologically and quantitatively with measurement of maximum diameter (MD). Results: Teratoma was observed in 10 of 11rats with tracheal defects and in 3 of 11 rats with abdominal subcutaneous tissue implants, respectively. Implanted iPS cells produced larger teratomas in tracheal defects than in the abdominal subcutaneous tissue. The average MD was 5.36 mm in the trachea and, 0.97 mm in the abdomen. Conclusions: The results of histological findings and quantitative measurements demonstrated that there were differences in teratoma formation according to the site of implantation. Further, we believe that it is necessary to carefully examanie the implantation site dependent differences for safety of iPS cell before proceeding with its clinical application.