Soft tissue related complications are quite frequent in limb lengthening. Muscle fibers may proliferate or regenerate after stretching injury over 10-20% of their original length. However, the cells which are engaged in this phenomenon are not confirmed yet. We chased the S-phase cells (phase for DNA replication) in the posterior leg muscle during limb lengthening by immunohistochemical technique. We lengthened the tibiae of fifteen New Zealand white rabbits. We divided them into three groups and each group is consisted of five rabbits. In group 1, we lengthened the left tibiae by 10% of their original length, in Group 2, 20%, and in group, 3, 25%, respectively, At the end of lengthening posterior muscles of lengthened left side and of controlled right side were fixed and processed for Immunohistochemical staining which could detect the incorporation of bromodeoxyuridine(BDU). Labelling index(LI:% of positively stained S-phase nuclei) of group 1 was zero. LI of groups for more than 20% lengthening (sum of group 2 and 3) was statistically significant.