PURPOSE: The effects of low-intensity pulsed ultrasound (LIPUS) on the expression of immediate-early genes (IEGs) in bone marrow stromal cells (BMSCs) were evaluated to elucidate the early cellular response to LIPUS. METHODS: Mouse ST2 BMSCs were treated with LIPUS (ISATA, 12-34 mW/cm2 for 20 min), then cultured at 37 °C. The expression levels of four IEGs (Fos, Egr1, Jun, and Ptgs2) and ERK1/2, a mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), were assessed using real-time quantitative PCR and Western blot analyses, respectively. RESULTS: A single exposure of LIPUS at an intensity of 25 mW/cm2 significantly and transiently increased the expression levels of all four IEGs, and the peak expression was detected at 30-60 min after LIPUS stimulation. LIPUS exposure also significantly increased the phosphorylation level of ERK1/2. U0126, an inhibitor of MAPK/ERK, significantly prevented LIPUS-induced expression of Fos and Egr1, but not that of Jun and Ptgs2. On the other hand, treatment of the cells with LIPUS did not affect cell growth or alkaline phosphatase activity, a marker of osteoblast differentiation. CONCLUSION: These results suggest that LIPUS exposure significantly induces expression of IEGs such as Fos and Egr1 via the MAPK/ERK pathway in ST2 BMSCs.