Spain's radiation oncology infrastructure has evolved in terms of both quantity and structure since the last analysis was carried out in 1999. The purpose of this paper is to describe the current facilities, basically consisting of external radiotherapeutic units, and to present a calculation model for estimating the number of irradiation units required to guarantee equitable cover for radiotherapeutic treatment. An electronic questionnaire was designed to be accessed via a link on the Spanish Association of Radiotherapy and Oncology Web page. The questionnaire provides details of the current state of Spain's radiotherapeutic facilities, and a descriptive study has been performed with analytical data. The scope of the study included all the centres providing oncology-radiotherapy in Spain. The units analysed encompassed the centres and irradiation units actively providing care in 2004. Ninety-seven centres were detected, in which 177 external radiotherapy units were registered. Forty-five of these were cobalt units and 132 were accelerators. The rate of radiotherapy use in Spain based on the best scientific evidence available and on its rate of use with cancer has been estimated at 61%. The number of external irradiation units available in 2004 (177) is clearly lower than the number desirable (266-316).