To test the hypothesis that extrinsic cells that infiltrate the devitalized patellar tendon (PT) synthesize type III collagen even in the environmental milieu of the native PT, we conducted the present experimental study using the rat in situ frozen-thawed PTs. Tissue culture showed no cell outgrowth from the tendons immediately after the freeze-thaw treatment. Analysis by RT-PCR showed that the expression level of type III procollagen mRNA in the frozen-thawed tendon was significantly higher than that in the sham-operated tendon at 6 and 12 weeks. Immunohistological findings showed positive type III collagen staining around cells that had infiltrated the necrotized tendon at 3, 6, and 12 weeks. In addition, the elastic modulus of the in situ frozen-thawed tendon at 6 weeks was significantly less than that of the sham-operated tendon. The present study indicates that extrinsic cells that had infiltrated the devitalized PT synthesized type III collagen at least for 12 weeks even in the environmental milieu of the native PT. These findings raised the question whether the increase in type III collagen of the PT graft after ACL reconstruction is really caused by "ligamentization," the adaptation of the PT graft to the ACL environment.