Abstract: The present invention relates to a method and apparatus for the uniform and simultaneous illumination by means of radiation of the interior surfaces of cavities for the purpose of observation and/or irradiation. In particular, such cavities may be not only hollow organ cavities, but also cavities in non-organic objects. The solution provides that the interior of the cavities is filled, at least in part, with a light scattering or dispersing medium and the dispersing medium is caused to shine or scatter light by means of one or a plurality of radiations or scattered light. With the integral irradiation used according to the present invention, two procedures are combined technologically, namely diagnosis and therapy of, for example, carcinoma in organ cavities which have been photosensitized by chemical substances, possibly on the basis of differences in decomposition of cancerous and normal tissue.