A device for visually combining patient image data from transillumination and/or tomographic imaging methods with object image data comprising video images includes an image display device and at least one video camera associated with the image display device as a portable unit. A computer-assisted navigation system can detect the spatial positions of the image display device and/or the video camera as well as the spatial positions of a part of the patient's body via tracking means. An input device can be provided on the image display device, which enables inputs for image-assisted treatment or treatment planning.