OBJECTIVE: The aim of this study was to investigate the effects of 670-nm, 780-nm, and 830-nm laser irradiation on cell proliferation of normal primary osteoblast (MC3T3) and malignant osteosarcoma (MG63) cell lines in vitro. BACKGROUND: Some studies have shown that laser phototherapy is able to stimulate the osteogenesis of bone tissue, increasing osteoblast proliferation and accelerating fracture consolidation. It has been suggested that laser light may have a biostimulatory effect on tumor cells. However, the mechanism by which the laser acts on cells is not fully understood. MATERIALS AND METHODS: Neonatal, murine, calvarial, osteoblastic, and human osteosarcoma cell lines were studied. A single laser irradiation was performed at three different wavelengths, at the energies of 0.5, 1, 5, and 10 J/cm(2). Twenty-four hours after laser irradiation, cell proliferation and alkaline phosphatase assays were assessed. RESULTS: Osteoblast proliferation increased significantly after 830-nm laser irradiation (at 10 J/cm(2)) but decreased after 780-nm laser irradiation (at 1, 5, and 10 J/cm(2)). Osteosarcoma cell proliferation increased significantly after 670-nm (at 5 J/cm(2)) and 780-nm laser irradiation (at 1, 5, and 10 J/cm(2)), but not after 830-nm laser irradiation. Alkaline phosphatase (ALP) activity in the osteoblast line was increased after 830-nm laser irradiation at 10 J/cm(2), whereas ALP activity in the osteosarcoma line was not altered, regardless of laser wavelength or intensity. CONCLUSION: Based on the conditions of this study, we conclude that each cell line responds differently to specific wavelength and dose combinations. Further investigations are required to investigate the physiological mechanisms responsible for the contrasting outcomes obtained when using laser irradiation on cultured normal and malignant bone cells.