Patients undergoing radiotherapy for rectal cancer are generally treated in a prone position, with a full bladder, to reduce the volume of normal bowel in the high-dose volume. This position is difficult to maintain, and is not consistently reproducible. This study evaluates the volume of bowel and dose received in the prone and supine positions in patients undergoing pre-operative rectal cancer chemoradiation. Using CT planning, 19 consecutive patients with rectal cancer with a full bladder underwent CT scanning first in the prone position and then immediately afterwards in the supine position. The planning target volume was outlined for the prone position and transcribed to the supine scan using pre-set criteria. The bladder and small bowel were outlined in both positions. Radiotherapy was planned using three-dimensional conformal planning, and treatment was delivered using three fields with multileaf collimators in two phases: phase I, pelvis 45 Gy/25 fractions; and phase II, tumour 9 Gy/five fractions. For both positions, the volume of bowel receiving doses in 5 Gy increments from 5-45 Gy was calculated using dose-volume histograms. At 5 Gy and 10 Gy dose levels, a significantly higher volume of bowel was irradiated in the supine position (p<0.001). At 15 Gy, it was marginally significant (p = 0.018). From 20-45 Gy, there was no significant difference in the volume of bowel irradiated with each 5 Gy increment. This study demonstrates that the volume of bowel irradiated at doses associated with bowel toxicity in concurrent chemoradiation is not significantly higher in the supine position. This position could be adopted for patients undergoing pre-operative rectal cancer chemoradiation.