Influence of MR-imaging on definition of treatment volume was studied prospectively in 43 patients undergoing radiotherapy for mediastinal malignancy (twelve Morbus Hodgkin, four non-Hodgkin-lymphoma, 26 lung cancer, one nephroblastoma). Conventional treatment planning using a simulator and all available information from axial CT-scanning and posterior-anterior and lateral radiographs was compared to a MRI-assisted procedure. Contours from coronary MR-sections acquired in treatment position were superposed onto the simulator radiograph using a subtrascope (MRI-simulation). MR-imaging using T1-weighted sequences resulted in excellent delineation of tumor masses from mediastinal fat, airways and vascular structures. The high soft tissue contrast allowed an exact and reproducible transfer of tumor contours onto the simulator radiograph. This led to changes in field configuration in 11/43 patients concerning mainly tumor extension infracarinally and in the caudal parts of the lung hili. Superiority of MRI-assisted simulation was noted as usefulness of axial CT-scanning to delineate margins was compromised in these areas by partial volume effects of tangentially represented structures and suboptimal contrasted vascular spaces.