OBJECTIVES:: To determine the recommended phase II dose of postoperative accelerated intensity modulated radiotherapy (IMRT) for prostate cancer. MATERIAL AND METHODS:: Step and shoot IMRT with simultaneous integrated boost (SIB) was delivered in 25 fractions over 5 weeks to patients with high risk resected prostate adenocarcinoma (stage pT3-4 and/or positive surgical margins). Pelvic nodes received 45 Gy at 1.8 Gy/fraction; dose escalation was performed only to the prostate bed (planned dose escalation: 56.8 Gy at 2.27 Gy/fraction, 59.7 Gy at 2.39 Gy/fraction, 61.25 Gy at 2.45 Gy/fraction, 62.5 Gy at 2.5 Gy/fraction). Dose-limiting toxicity (DLT) was any grade ≥ 3 acute toxicity (RTOG score). RESULTS:: Twenty-five patients were treated: 7 patients at the 56.75 Gy dose level, 6 patients at each subsequent dose level. Pathologic stages were: pT2c: 2; pT3a: 11; pT3b: 12; pN0: 22; pN1: 3; R0: 7; R1: 18. Median follow-up time was 19 months (range: 6-36 months). No patient experienced DLT. Grade 1-2 acute rectal and urologic toxicity was common (17 and 22 patients, respectively). CONCLUSIONS:: The recommended dose was 62.5 Gy in 2.5 Gy/fraction. Postoperative hypofractionated IMRT SIB for prostate cancer seemed to be well tolerated and could be tested in phase II studies.