Low level laser therapy (LLLT) has been used as a treatment in many clinical conditions and it seems to be able to reduce post-injury inflammatory processes and to accelerate soft and hard tissue healing. It has been demonstrated that LLLT stimulates mitochondrial respiration, increases the synthesis of DNA, RNA and regulatory proteins, promoting cell proliferation, in normal and malignant cells. However, the mechanism by which LLLT acts on cells is not fully understood. Then, in order to progress our understanding of the physiological processes and clinical parameters involved in the field of laser therapy, and to determine the responses of bone malignant cells to laser irradiation, we investigated the dose-response effects of 670nm, 780 nm and 830 nm laser on cell proliferation of 2 different osteosarcoma (MG63 and U2OS) cell lines in vitro. Methods: Cell lines were grown and passaged using standard aseptic tissue culture protocols. For laser irradiation, the wells of 96-well plates were seeded with 1x104 cells in fresh growth medium, then incubated for 24hr at 37oC in 5%CO2. A single dose of irradiation was performed (670nm, 780 nm and 830 nm, 30 mW) at the intensities of 0.5, 1, 5 and 10 J/cm2. Cell proliferation was then assessed 24hr after treatment by colorimetric spectrophotometry. Statistical analysis: Data were described as means and standard deviation (SD) of the percentage. A 2-way ANOVA test was used to assess the significance of differences between the percentage of increase or decrease of irradiated groups compared with negative (untreated) controls, and the Duncan䳠test to identify the differences. A p value of 0.05 was set for determining statistical significance. Results: It was observed that MG63 osteosarcoma cell proliferation increased significantly after 670nm (at 5 J/cm2) and 780nm laser irradiations (at 1, 5 and 10 J/cm2, p= 0.05), but not alter after 830nm laser irradiation. In contrast, U2OS cell proliferation was found significantly increased after the irradiation with all laser wavelength used, at all doses (p= 0.05). Conclusions: The results of the present study suggest that laser irradiation may have stimulatory effects on osteosarcoma cell proliferation. However, these data highlight that different osteosarcoma cell lines had different responses to different types of laser using specific irradiation parameters. Further investigations are required to investigate possible response mechanisms that may explain the outcomes obtained when examining laser irradiation of cultured cells. Such future studies will undoubtedly contribute to a better understanding of the safety and efficacy of LLLT in clinical oncology. Key words: cell proliferation, low level laser therapy, osteosarcomas.