Latest developments in radiation therapy such as IGRT (image guided radiation therapy) and IMRT (intensity modulated radiation therapy) promise to spare organs at risk by applying better dose distribution on the tumor. For any effective application of these methods the exact positioning of the patient and the localization of the exposured organ is crucially. With respect to the filling of rectum and bladder the prostate can move several millimeters up to centimeters. That implies the need of daily determination and correction of the position of the prostate before irradiation. We build a system which uses 3D US at both sites, the CT room and the intervention room and applied a 3D/3D USUS registration for fully automatic repositioning. In a first step an appropriate preprocessing of the US images is necessary. We implemented an importance image filter process to improve the following registration success. For the 3D3D registration five different object functions were implemented. To find the object function which fits best for the particular patient three 3D US images were taken at the CT site and an US registration error was calculated. The most successful object function was then applied at the treatment site.The US registration error was found to be 3.48 ± 2.32 mm (eight patients) with resect to the Mutual Information metric by Mattes. For complete repositioning the distance error amounted to be 5.0 ± 3.1 mm (four patients).© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.