A prospective study was undertaken to evaluate the improvement in rectal cancer radiation treatment achieved with the implementation of target delineation for conformal radiotherapy, replacing conventional technique using standard radiological anatomy for target volume definition. In 10 patients receiving preoperative pelvic irradiation for rectal cancer, a 3-field technique was designed by a 3-dimensional (3D) planning system. Two plans were simulated for each patient, one with the fields designed in the conventional way based on radiological anatomy, and the other with the fields designed on the basis of a computed tomography (CT) delineated planning target volume (PTV). A total dose of 45 Gy in 25-daily fractions of 1.8 Gy in 5 weeks was planned. Dose-volume histograms (DVHs) of PTV, small bowel, anal sphincter, and urinary bladder were analyzed to compare plans. The minimum, maximum, and mean dose in the PTV and in critical organs were also evaluated. The inhomogeneity coefficient (IC) and the target coverage (TC) were calculated. The normal tissue complication probability (NTCP) for each organ at risk (OAR) was determined. NTCP for small bowel and urinary bladder was not statistically different, while the PTV coverage was significantly lower with conventional treatment relative to conformal treatment (median IC=7.2, median TC=0.91 vs. median IC=0.14 and median TC=1, p<0.005). The 3D conformal treatment plan in preoperative radiotherapy for rectal cancer improves target coverage without significantly affecting small bowel and urinary bladder NTCP.