Although risedronate, a nitrogen containing bisphosphonate (BPs), strongly inhibits bone resorption by enhanced apoptosis of osteoclasts, its mechanism remained unclear. In this study, we investigated the molecular mechanism of risedronate-induced apoptosis of U937 cells, with a focus on extracellular signal-regulated kinase 1/2 (ERK 1/2) and protein kinase B (Akt) pathways, mitochondria-mediated apoptosis, and the effect of disruption of the actin cytoskeleton. Risedronate facilitated the relocation of Ras from membrane to cytosol through inhibited isoprenylation. Accordingly, risedronate suppressed the phosphorylation of ERK 1/2, a downstream survival signaling kinase of Ras, affected the intracellular distribution of Bcl-xL, and induced the mitochondrial membrane depolarization, cytochrome c release, activated caspase cascade and DNA fragmentation. The risedronate-induced apoptosis was effectively suppressed with cyclosporine A plus trifluoperazine, potent inhibitors of mitochondrial membrane permeability transition (MPT). The risedronate-induced apoptosis was independent of Akt, another cAMP-dependent survival signaling kinase. Risedronate facilitated dephosphorylation of Bad at Ser112, an ERK phosphorylation site, but not at Ser136, an Akt phosphorylation site. All of these apoptosis-related changes induced by risedronate were strongly suppressed by cytochalasin B, an inhibitor of actin filament polymerization. These results indicate that risedronate-induced apoptosis in U937 cells involves Ras/ERK, but not Akt signaling pathway, and is dependent on MPT, and that disruption of the actin cytoskeleton inhibits the risedronate-induced apoptosis at its early step.