There is provided an apparatus for visualizing a region of interest (ROI) in a Computer Aided Diagnosis (CAD) system. The apparatus includes: an image receiver configured to receive images; an ROI acquirer configured to acquire the ROI from a current image; and an ROI visualizer configured to, in response to acquisition of the ROI from the current image, output visualization information for visualizing the ROI acquired from the current image based on a change between the ROI acquired from the current image and an ROI acquired from a previous image.