To investigate the effect of an endorectal balloon (ERB) on anal wall (Awall) and rectal wall (Rwall) doses in high-dose post-prostatectomy intensity-modulated radiotherapy (IMRT).For 20 patients, referred for salvage IMRT after prostatectomy for prostate cancer, two planning CT-scans were performed: one with and one without an air-filled ERB. A planning target volume (PTV) was defined, using international guidelines. Furthermore, the Awall and Rwall were delineated. In both the scans, IMRT plans were generated with a prescribed dose of 70Gy. The mean dose (D mean ), maximum dose, minimum dose, and volumes exposed to doses ranging from ⩾20 to ⩾70Gy (V 20 –V 70 ) to the Awall and Rwall were calculated. Finally, inner Rwall surface areas exposed to doses ranging from ⩾20 to ⩾70Gy (A 20 –A 70 ) were calculated. Dose-parameters were compared between plans with and without ERB.All Awall parameters, except V 70 , were significantly reduced by the ERB with an overall D mean reduction of 6Gy. Absolute reductions in dose–volume parameters varied from 5% to 11%. Significantly reduced Rwall V 30 , V 40 , and A 40 were observed with ERB, irrespective of the target volume size.ERB application significantly reduces Awall and to a lesser degree Rwall doses in high-dose post-prostatectomy IMRT.