The present rapid worldwide expansion of particle radiotherapy services will inevitably have an impact on clinical practice within the UK. The most recent results of developmental trials using protons and carbon ions are impressive, with high cure rates and little or no functional normal tissue changes and a very low level of serious treatment-related morbidity. The potential numbers of patients that will demand or are referred for treatment abroad are estimated, assuming different rates of change and treatment capacities with time. Even if the maximum demand were to be under 10% of all patients presently treated by radiotherapy, significant numbers (amounting to several thousand patients per year) may be advised to seek treatment abroad between 5 and 10 years from now. The gap between overall demand and the estimated numbers could be partly, although substantially, filled by the establishment of a single large UK facility. Should demand increase beyond the estimated level, for example due to improved screening of cancer, then a network of UK particle radiotherapy centres will be required.