Abstract: A method for determining the position of a patient's organ with respect to at least two imaging devices includes a step of making at least one three-dimensional first image of an organ of a patient giving a first coordinate system and of a surface of the organ or skin region using a first imaging device without fixing any mark to the patient, placing the patient in an operation site having a second coordinate system and determining a first position of a second imaging device with respect to the second coordinate system of the operation site. In addition, the method includes the steps of making a second image with the second imaging device, matching the first and second images and providing a third device having a third coordinate system. The third device is positioned with respect to the second image of the second device which thereby positions the third device with respect to the first image of the first imaging device.