It is common practice to utilize CT scans in place of patient contours for computerized treatment planning. Our experience, in particular with pelvic patients, is that setting up the patient for a CT scan using landmarks from a radiation therapy simulator does not always produce accurate field margins. In general, the treatment field center as produced by the CT scan is superior to the center as defined by the simulator film. We have determined this problem to be caused by simulator table sag as compared to the CT. We offer suggestions to correct the problem.