Introduction: Prone position can be used for the plannig of adjuvant radiotherapy after breast conservative surgery. It is aimed to deliver less irradiation to lung tissue and cardiac tissue in the prone position. Materials and Mcthods: Two cases of left breast carcinoma in stages Tl, NO, MO were taken for three-dimensional conformal radiotherapy planning after breast conservative surgery. The first case had moderate size of breast tissue. They were positioned with breast ring in the supine position and with foam which had a hole in order to allow the left breast tissue coming down in to prone position. Results: Isodose distributions and dose volüme histograms of the cases in the prone position were compared with the ones in the supine position. We have observed less irradiation given to the left lung tissue, cardiac tissue and right lung tissue in the prone position than in the supin position. Discusion: Three-dimensionel conformal radiotherapy planning was used to provide less normal tissue complication and high lokal control in both of the supine and prone position. Large breasted women appeared to benefit most from the planning in the prone position.