The effects of carbon ions on bone volume were studied by histological and morphometrical methods. Anesthetized rats irradiated with three different single doses (15, 22.5, or 30 Gy) of either carbon ions or gamma rays were sacrificed (5 rats/group) at 10 time points together with non-irradiated controls. Carbon ion-induced bone responses were qualitatively and quantitatively different from those induced by gamma irradiation at the same dose level. Irradiation with carbon ions resulted in a dose-dependent increase in bone volume, while gamma irradiation induced loss of bone volume, which was found to be independent of dose in the range studied over the same time course. After irradiation with carbon ions, bones showed thickening of the trabeculae, and the bone marrow became fibrous with fewer vessels. Carbon ion irradiation showed a greater effect on the bone-absorbing capability of osteoclasts than gamma irradiation. These observations suggest that irradiation with carbon ions may produce differential modulation of radiation-induced growth factor expression.