Some authors proposed some mechanisms by which the diode laser relieves pain: Pawaret al. 16 and Bjordalet al. 18 found that the diode laser acts on chronic pain and has an anti-inflammatory effect by decreasing PGE2, bradykinin, histamine, acetyl choline and serotonin, also the diode laser was proved to decrease the production of substance P. Our results concerning postoperative pain in the cases of chronic apical periodontitis, when treated with diode laser, showed promising results. Here, TAC1 is linked to chronic apical periodontitis.