LOCATE, a new computer software program, was used to outline tumor margins on scanned projection radiographs (ScoutView). This information was transferred to simulator radiographs and the results compared with tumor margins determined by all other methods. Twenty-two patients with various types of tumors in different parts of the body were studied with computed tomography and then therapy planned without the benefit of images from this program. When program information was made available, it stimulated a change in the radiation therapy plan in 18 cases (82%). Phantom studies demonstrated that magnification inherent in the program images was not significantly different from that in simulator radiographs. Program-generated tumor margins on ScoutView images were shown to accurately reflect tumor margins outlined on cross-sectional computed tomographic images as long as no patient motion occurred on the table during the study. LOCATE provides a useful means of tumor localization for therapy planning in many cases.