['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 70 Gy. The mean dose (D(mean)), maximum dose, minimum dose, and volumes exposed to doses ranging from ≥ 20 to ≥ 70 Gy (V(20)-V(70)) to the Awall and Rwall were calculated. Finally, inner Rwall surface areas exposed to doses ranging from ≥ 20 to ≥ 70 Gy (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 6 Gy. 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.'] Does endorectal balloon reduce anorectal doses in post-prostatectomy intensity-modulated radiotherapy?