The Heavy Ion Medical Accelerator in Chiba (HIMAC) is the world’s first heavy ion accelerator complex dedicated to medical use in a hospital environment. Heavy ions have superior depth‐dose distribution and greater cell‐killing ability. In June 1994, clinical research for the treatment of cancer was begun using carbon ions generated by HIMAC. Until August 2002, a total of 1,297 patients were enrolled in clinical trials. Most of the patients had locally advanced and/or medically inoperable tumors. Tumors radio‐resistant and/or located near critical organs were also included. The clinical trials revealed that carbon ion radiotherapy provided definite local control and offered a survival advantage without unacceptable morbidity in a variety of tumors that were hard to cure by other modalities. © 2003 American Institute of Physics