The cost of delivering a radiotherapy treatment on a linear accelerator is calculated. Items included in the cost are: cost of the accelerator itself, a simulator, a treatment planning computer, the treatment room, and the salaries of the radiographers and physicists concerned, as well as machine operating costs and interest charges. The cost of a beam direction shell is also calculated. The different costs are reduced to statements of cost per year (see Table IV for summary), and from these the cost of a course of treatment for a patient is arrived at on the basis of 800 patients treated per year. The cost comes out at pounds 137, plus pounds 70 for a beam direction shell, if one is used. Because the capital costs of radiotherapy treatment equipment are very substantial, it is often assumed that radiotherapy is a very expensive form of treatment. An analysis of treatment cost is presented, initially in terms of cost per year for equipment, special buildings, and staff at 1981 prices. Reduction of these figures to mean treatment cost per patient is based on the DHSS guideline for provision of radiotherapy services (HC(78)32), which suggests a figure of 800 patients to be treated per year as the number justifying the provision of a linear accelerator.