In June 1994, the world's first clinical center offering carbon ion radiotherapy opened at the National Institute of Radiological Science (NIRS), Japan. Among several types of ion species, carbon ions were chosen for cancer therapy because they were judged to have the most optimal properties in terms of superior physical and biological characteristics. As of March 2010, 5,196 patients have been registered for carbon ion radiotherapy. Clinical results have shown that carbon ion radiotherapy has the potential to provide a sufficient radiation dose to the tumor, while having acceptable morbidity in the surrounding normal tissues. Tumors that appear to respond favorably to carbon ions include locally advanced tumors as well as histologically non-squamous cell tumor types such as adenocarcinoma, adenoid cystic carcinoma, malignant melanoma, hepatoma, and bone/soft tissue sarcoma. By taking advantage of the unique properties of carbon ions, treatment with small fractions within a short treatment period has been successfully carried out for a variety of tumors. This means that carbon ion radiotherapy can offer treatment for larger numbers of patients than is possible with other modalities over the same time period.