PURPOSE: To investigate the effect of alendronate on the expression of Id genes in osteoblast differentiation. METHODS: C2C12 cells were treated with alendronate for various concentrations and time periods. For evaluation of alendronate-induced osteoblast differentiation in C2C12 cells, alkaline phosphatase (ALP) activity was measured. The expression of osteoblast differentiation markers such as ALP, type-1 collagen (Col 1), and osteocalcin (OCN), and the expression of Id-1 and Id-2 were measured by RT-PCR. In order to understand the mechanism underlying the regulation of Id genes, the promoter region of the Id-1 gene was identified. Database analysis of the promoter region for Id-1 using known consensus sequences identified several putative response elements, including CCAAT/enhancer-binding protein beta (C/EBPbeta). RESULTS: Alendronate treatment significantly increased not only ALP activity but also the expression of ALP, Col 1, and OCN, Id-1 and Id-2. C/EBPbeta and alendronate cooperatively increased the promoter activity and expression of Id-1. CONCLUSIONS: These results suggest that C/EBPbeta-mediated Id-1 transcriptional activation may regulate alendronate-induced osteoblast differentiation of C2C12 cells.