Clinical studies have revealed that low-energy laser irradiation facilitates alleviation of pain or wound healing. However, basic studies have demonstrated that irradiation using low-energy laser such as He-Ne laser modulates various biological processes. Some reports found that He-Ne laser irradiation activated the cell cycle and stimulated cell proliferation activity, while others suggested that it indicated inhibitory behavior. Further elucidation of the photo-chemical effects of low-energy laser irradiation on cultured cells is required. The present report reviews our studies on the effects of low-energy laser irradiation with He-Ne laser.<br>We have also been examining the effects of He-Ne laser irradiation on the proliferation activity of rat bone-marrow-derived osteoblastic cells cultured on TiO₂ discs. The results suggest that He-Ne laser irradiation stimulates both the proliferation and differentiation capacity of the cells. Therefore, He-Ne laser irradiation around an implant hole prior to pegging an implant body might promote healing of bone tissue and contribute to an initial fixation of a dental implant, leading to an increase in the graft survival rate.<br>We also examined the effects of He-Ne laser irradiation on the growth of mouse colonic-carcinoma-derived tumor cells. The results showed that brief irradiation stimulated proliferation of the cells, whereas prolonged irradiation caused cytostatic activity of the cells.<br>These studies revealed that the effects of low-energy laser irradiation on various cells differ according to the kind of laser, irradiation time, and amount of energy as well as the kind of cells. Further studies on the photo-chemical effects of low-energy laser irradiation are necessary.