Prostate cancer is a considerable health problem in USA and Europe. In USA, prostate cancer is the second highest cause of cancer death in men (Keeley FX et al, 1998). Radiotherapy is an established standard treatment modality for localized prostate cancer. Radiation dose response relationship with tumor control has been demonstrated (Pollack A et al 1997). However, the delivered radiation dose is limited by proximity of critical normal structures (i.e. urinary bladder and rectum). Using conventional radiation therapy techniques, higher radiation dose is associated with higher incidence of treatment-related toxicity, with relatively high frequency of local relapse This may be due to underestimation of the true volume of the prostatic target, such that a geographic miss occurs, or due to the fact that the radiation dose delivered using the conventional approach may be too low to eradicate all tumor cells. Recently, 3D-CRT and IMRT is the product of advances in radiotherapeutic technology to deliver radiation more precisely to the tumor while limiting the dose to the surrounding organ at risk (OAR). Why IMRT? Compared to conventional 3D-CRT, IMRT improves conformality of high dose radiotherapy delivery. This study was designed to document the acute toxicity of the most recently developed radiation therapy techniques namely the 3-DCRT using multileaf collimators (MLCs with 6 fields technique) compared to the IMRT using step and shoot technique (5 fields). In addition, the results were to be compared to that of retrospectively reviewed matched archival cases treated at Saint Louis hospital - Paris between April 1999 to August 2002 with 3-DCRT using customized blocks (4 fields box technique). The results revealed no gastrointestinal or genitourinary grade III acute toxicity scored according to the acute RTOG morbidity scale in both prospective groups. While the acute genitourinary and gastrointestinal grade III toxicity in the retrospective group was 2.4% and 4 % respectively. In order to study the impact of rectal volume on the development of late rectal toxicity different dosimetric parameters had been evaluated in the retrospective group. This study revealed that the absolute rectal volume <15 CC and 35 CC at 70 Gy and 65 Gy, respectively, were the most statistically significant predictive factors for late rectal toxicity. These two predictive factors were taken into consideration in the treatment planning of both prospective groups while contouring the rectum. Based on the data analysis in the present study dose escalation to 76-78 Gy is well tolerated and was not associated with grade III acute genitourinary or lower gastrointestinal toxicity. Hence, we would suggest that Three-dimensional -conformal radiotherapy (3D-CRT) is now considered to be standard modern treatment for localized prostate cancer. 3D-CRT and IMRT improved physical dose distributions compared with current 2D treatment planning. IMRT represents a new paradigm in radiation treatment planning and delivery for treatment of prostate cancer. However, clinical trials with long term follow up will be necessary to define more clearly the true extent of improved tumor control and reduction in normal tissue complications.