The work described here forms part of a program of research into the application of direct volume rendering methods to the visualization of radiation therapy plans. The full program covers a number of related topics, including the investigation of fast parallel algorithms for interactive rendering, the design of special protocols for communication between low-cost workstations and a parallel computer used to store data and generate images, and research into direct manipulation techniques for interaction with volume data. In this paper, however, we focus on the difficulties of visualizing 3D volume data, and we report the results of preliminary experiments designed to evaluate the utility of stereoscopic displays for this purpose. The remainder of the paper is organized as follows. In the next section we outline the specific problem we are trying to solve: the interactive visualization and manipulation of radiation therapy plans. We introduce, briefly, direct volume rendering methods outlining the approach we have taken and the reasons for this. Next, we address some of the problems which arise, and how stereoscopic displays may help to alleviate these. We then describe our experiments and their results, concluding with a discussion of their significance.