World wide, in 1996, there are 17 operating proton therapy centers treating patients for both benign and malignant disease. Most centers use accelerators originally designed for physics research and adapted for medical use; only one center uses an accelerator designed specifically to operate in a hospital setting. Five centers with proton beam energies of <75 MeV, treat tumors of the eye only. By July 1996, ∼17,737 patients world wide have been treated with proton beams and approximately half of these patients were treated from 1990–1996. This recent increase in the number of patients treated reflects the increase in the number of operating facilities in this time period.