To assess acute gastrointestinal (GI) and genitourinary (GU) toxicities in patients with localized prostate cancer treated with a sequential dose escalation hypofractionated intensity-modulated radiotherapy (IMRT) study using two different delivery methods. Since 2003, 88 and 48 patients were sequentially treated to 56 Gy and to 60 Gy (4 Gy/fraction twice weekly), respectively. IMRT with 6 MV beams was delivered with five fields in Geneva and with nine in Barcelona. Acute GI and GU side effects were scored weekly during treatment and 6 weeks after treatment completion using the Radiation Therapy Oncology Group (RTOG) toxicity scale. Clinical, technical, and dosimetric parameters were analyzed in order to assess for a potential correlation with toxicity. Grade 1-2, GU and GI toxicities during and 6 weeks after treatment completion were 64%, and 24%, and 35% and 12%, respectively. Only one Grade 4 GU toxicity, consisting of transitory urinary obstruction, was observed. Patients treated to 60 Gy in Geneva presented a higher rate of Grade 1-2 GU toxicity (p = 0.01), while patients treated to both 56 and 60 Gy in Barcelona presented a higher Grade 1-2 GI toxicity (p = 0.02). A lower rate of rectal toxicity was observed in the subgroup of 22 patients treated with a rectal balloon (p = 0.02). The use of androgen deprivation therapy was associated with a higher rate of Grade 1-2 GU toxicity after the end of the treatment (p = 0.02). Dose escalation with either 14 _ 4 Gy or 15 _ 4 Gy delivered with two different IMRT techniques is feasible and is associated with a tolerable acute toxicity.