Background and purpose: In order to apply highly conformal dose distributions, which are characterized by steep dose fall-offs, it is necessary to know the exact tumor location and extension. This study aims at evaluating the impact of using combined CT-MRI images in organ delineation compared to using CT images alone, on the clinical results. Materials and methods: For 10 prostate cancer patients, the respective CT and MRI images at treatment position were acquired. The CTV, bladder and rectum were delineated using the CT and MRI images, separately. Based on the CT and MRI images, two CTVs were produced for each patient. The mutual information algorithm was used in the fusion of the two image sets. In this way, the structures drawn on the MRI images were transferred to the CT images in order to produce the treatment plans. For each set of structures of each patient, IMRT and 3D-CRT treatment plans were produced. The individual treatment plans were compared using the biologically effective uniform dose ( D ) and the complication-free tumor control probability (P+) concepts together with the DVHs of the targets and organs at risk and common dosimetric criteria. Results: For the IMRT treatment, at the optimum dose level of the average CT and CT-MRI delineated CTV dose distributions, the P+ values are 74.7% in both cases for a DCTV of 91.5 Gy and 92.1 Gy, respectively. The respective average total control probabilities, PB are 90.0% and 90.2%, whereas the corresponding average total complication probabilities, PI are 15.3% and 15.4%. Similarly, for the 3DCRT treatment, the average P+ values are 42.5% and 46.7%, respectively for a DCTV of 86.4 Gy and 86.7 Gy, respectively. The respective average PB values are 80.0% and 80.6%, whereas the corresponding average PI values are 37.4% and 33.8%, respectively. For both radiation modalities, the improvement mainly stems from the better sparing of rectum. According to these results, the expected clinical effectiveness of IMRT can be increased by a maximum ΔP+ of around 13.2%, whereas of 3DCRT by about 15.8% when combined CT-MRI delineation is performed instead of using CT images alone. Conclusions: It is apparent that in both IMRT and 3D-CRT radiation modalities, the better knowledge of the CTV extension improved the produced dose distribution. It is shown that the CTV is irradiated more effectively, while the complication probabilities of bladder and rectum, which is the principal organs at risk, are lower in the CT-MRI based treatment plans.