We investigated the effects of ethane-1-hydroxy-1, 1 diphosphonate (EHDP) on Dunn osteosarcoma cells in vivo and in vitro. In in vivo study, an increase of tumor volume was significantly suppressed in the EHDP administered groups compared with the control group. Histologically, Dunn osteosarcoma cells' viability was maintained after EHDP administration. However, fatty degeneration of tumor tissue was suspected in two of eight mice in the 5.0 mg/kg EHDP administered group. In vitro study, EHDP inhibited DNA synthesis and induced morphological changes, such as pycnotic cells. These findings show that the growth of Dunn osteosarcoma cells is inhibited by EHDP.