Although Er:YAG laser treatment has been reported to enhance wound healing, the mechanism of action is unclear. Activation of periodontal ligament cells has the potential to accelerate wound healing in periodontal treatment. The purpose of this study was to evaluate the effects of low-level Er:YAG laser irradiation (LLLT: Low reactive Level Laser Therapy) for transforming growth factor-β (TGF-β) in cultured human periodontal ligament (hPDL) cells.<br>Materials and Methods: After irradiation of hPDL cells with Er:YAG laser (50mJ, 10pps, 10sec), alkaline phosphatase (ALP) activity was determined following the addition of dentin phosphoprotein (DPP), dentin sialoprotein (DSP), tyrosine-rich amelogenin polypeptide (TRAP), and 20kDa amelogenin. Furthermore, the activity of TGF-β by Er:YAG laser irradiation was evaluated by determining ALP activity in hPDL cells with or without the addition of SB431542 (SB), which is a specific inhibitor of TGF-β. Cell proliferation was determined by MTS assay, and the hPDL cells were observed and photographed using a phase-contrast microscope. Thereafter, the activation of TGF-β was investigated by determining ALP activity in hPDL cells due to the latent TGF-β after Er:YAG laser irradiation.<br>Results: When compared with the non-irradiation group, the Er:YAG laser radiation group showed a tendency to give higher values of ALP activity, and no difference was found between the added physiologically active substances. Moreover, the ALP activity was dramatically increased under the condition without the addition of SB. MTS assay showed that cell numbers of the laser irradiation group were higher than those of the non-irradiation group at day 3. A phase-contrast micrograph of cultured hPDL cells, taken 2 days after Er:YAG laser irradiation, showed irregularly shaped cells in comparison to those of the non-irradiation group. Addition of Er:YAG laser irradiated latent TGF-β to hPDL cells did not result in increased ALP activity.<br>Discussion: The present study showed a tendency for ALP activity to increase in hPDL cells by irradiating with Er:YAG laser. Therefore, it was suggested that Er:YAG laser irradiation might promote hard tissue formation. Furthermore, this enhancement of ALP activity was considered to be correlated with TGF-β, because the ALP activity was remarkably reduced by the addition of SB, which is a specific inhibitor of TGF-β. However, latent TGF-β was not directly activated by Er:YAG laser irradiation, because an increase of ALP activity in hPDL cells was not found by the addition of laser irradiated-latent TGF-β.