Real-time volume rendering of medical image datasets on commercial hardware became possible in 1993. We have developed an application, SeeThru, that allows real-time volume visualization under the interactive control of the physician. This ability enables the physician to look inside of the patient's body to visually comprehend the information from radiological procedures, resulting in improved treatment planning. We report on preliminary results from two areas: (1) cardiothoracic surgical planning from spiral computed tomography (CT) and (2) staging of breast cancer from magnetic resonance imaging (MRI). We compared different rendering methods (projection, maximum intensity projection, opacity blended, and opacity combined with gradient blended) and chose opacity blending as the most effective for both applications. In cardiothoracic surgical planning experiment we found the ability to interactively control and view 3D direct volume visualizations resulted in improvements in surgical plans and in the surgeon's confidence in the plan. In the MR breast experiment we found that 3D visualization of the subtraction images improved comprehension and identification of tumor lesions difficult to appreciate on mammograms. Overall, we believe that interactive, real-time volume rendering significantly adds to clinical understanding and improves treatment planning for the patient.