Infiltrating leukocytes from the periphery have been previously reported to be associated with the development of neuropathic pain in animal models.  In the current study, we used a mouse BALB/c spinal nerve L5 transection (L5Tx) neuropathic pain model and FACS analyses to investigate the role of peripheral and central nervous system (CNS) leukocyte populations in the etiology of persistent pain. We observed significant leukocyte infiltration (CD45^hi) in the lumbar spinal cord region at day 7 post-L5Tx compared to animals that underwent sham surgery.  These infiltrating leukocytes were found to be predominantly CD4⁺ T lymphocytes.  Other types of leukocytes, including CD8⁺ T lymphocytes, B lymphocytes, natural killer cells and macrophages were not detectable at this time point.  The activation status of infiltrating CD4⁺ T lymphocytes were further determined by surface markers: CD69 (leukocyte early activation marker), CD45RB (CD45RB^hi indicating naïve T lymphocytes, CD45RB^lo indicating effector/memory T lymphocytes) and CD154 (ligand for CD40, expressed primarily on activated CD4⁺ T lymphocytes)].  The CD4⁺ T lymphocytes expressed an activated/effector phenotype.  Peripheral activation of CD4⁺ T lymphocytes post- L5Tx and sham surgery was also detected in spleen and local lumbar lymph nodes, with no significant differences observed between groups.  In a separate study, athymic nude BALB/c mice, which lack mature functional T lymphocytes, developed significantly less mechanical allodynia after L5Tx, a level comparable with the wild type BALB/c mice post-sham surgery.  In summary, these results demonstrate that spinal cord infiltrating CD4⁺ T lymphocytes vs. peripheral T lymphocytes play an important role in the development and maintenance of spinal nerve injury-induced neuropathic pain.  The underlying mechanisms of the role of CNS infiltrating CD4⁺ T lymphocytes in neuropathic pain development are currently being investigated.