There are a variety of clinical situations in which patients undergoing radiation therapy can benefit from being treated in an upright position. The authors describe a new design for a treatment chair to assist in accomplishing this task. The present chair differs from previous designs in that it can be used with existing radiotherapy simulators as well as treatment units and that it permits isocentric setup and treatment of tumors either at the nominal source-to-axis distance (SAD) of a machine or at extended distance. This design permits treatment of mediastinal tumors as well as those of the head and neck using a variety of field arrangements including AP-PA, opposed laterals, and multiple obliques. The seat is designed on the "tool platform" principle. A wide variety of devices can be attached onto it to ensure accurate and reproducible, yet comfortable, patient positioning.