['The objective of this study was to investigate the effects on the proliferation and osteogenic differentiation of rat mesenchymal stem cells (MSCs) by using red-light light-emitting diode (LED) irradiation.', 'Low-level light irradiation (LLLI) has been shown to enhance proliferation and cytokine secretion of a number of cells. MSCs are capable of regenerating various mesenchymal tissues and are essential in supporting the growth and differentiation of hematopoietic stem cells within the bone marrow.', 'Rat bone marrow MSCs were treated with single or multiple doses of LLLI from an LED array (630 nm) at the irradiances of 5 and 15 mW/cm(2), and radiant exposures of 2 and 4 J/cm(2). The proliferation, clonogenic potential, and osteogenic differentiation of MSCs were evaluated after illumination.', 'The growth of MSCs was enhanced by red-light LLLI, and the effect became more obvious at low cell density. A single dose of LLLI led only to a short-term increase in MSCs proliferation. A maximal increase in cell proliferation was observed with multiple exposures of LLLI at 15 mW/cm(2) and 4 J/cm(2). The number of colony-forming unit fibroblasts increased when cells were illuminated under the optimal parameter. During osteogenesis, significant increases (p < 0.01) in both alkaline phosphatase and osteocalcin expressions were found in the MSCs that received light irradiation.'] Does red-light light-emitting diode irradiation increase the proliferation and osteogenic differentiation of rat bone marrow mesenchymal stem cells?