Automated coronary border detection techniques sometimes fail to accurately identify vessel borders, in part because they identify the left and right borders independently. We developed a method for simultaneous detection of both coronary borders that is based on three-dimensional graph searching principles. The simultaneous border detection method and our previously reported conventional method were applied to 29 coronary images, of which 19 were selected because conventional methods might be expected to have difficulty due to the presence of poor contrast, branching vessels, or nearby or superimposed structures. Coronary borders identified by the two methods were visually compared. In the 19 difficult images, simultaneous border detection yielded superior results in 7 images and equivalent results in 12 images. Superior or equivalent results were obtained in the remaining 10 typical images. In a set of 43 uncomplicated images, minimal lumen diameters derived using simultaneous border detection correlated well with diameters derived using conventional border detection (r = 0.97), diameters obtained from observer-defined borders (r = 0.91), and with diameters obtained using the Brown-Dodge quantitative coronary arteriography method (r = 0.85). Simultaneous detection of left and right coronary borders provides improved accuracy in the detection of vessel borders in difficult coronary angiograms.