Conformational radiotherapy is more and more used. It seems that this kind of treatment requests more manpower and material than 2D techniques. The purpose was to estimate its cost. We first tried to estimate the development of 3D radiotherapy. Then we calculated its relative cost compared to classical radiation therapy. To estimate relative costs, we first used the official catalogue. Then we reported results of a study performed in 1998 in two different radiotherapy departments. We conclude that 3D treatment are more expensive. The incremental cost is mainly related to that of dosimetry.