Following anterior cruciate ligament (ACL) reconstruction with the middle third part patellar tendon, the graft undergoes histological rearrangement due to biomechanical action, which transforms it into a structure similar to the normal ACL. The purpose of our study was to make a qualitative and quantitative histological evaluation, by transmission electron microscopy (TEM), of the neoligamentization process of a bone-patellar tendon-bone (BTPB) graft used as pro-ACL at different follow-up times. We analysed the ultrastructure of collagen fibrils by focusing on their size and distribution with respect to a normal patellar tendon and a normal ACL used as controls. Our results showed that up to 24 months follow-up, progressive ultrastructural changes towards the normal ACL were observed. At longer times after surgery (48 and 120 months) no further changes were evident and the ultrastructure showed a marked reduction in large fibrils, which was typical of the control patellar tendon, and a significant increase in small fibrils. The ultrastructure seemed to combine fibrils from two different morphological units. The BPTB graft used as ACL underwent a transformation process for up to two years. After that period the transformation ceased and for ten years failed to reach the ultrastructural aspect of a normal ACL. However, from an architectural point of view the graft was slowly transformed into a structure similar to ACL with respect to the different mechanical stresses the ligament has to sustain.