Colonoscopy is an endoscopic technique that allows a physician to inspect the inside of the human colon. Although millions of colonoscopic procedures are performed annually, an objective method that estimates how much effort was undertaken to insure maximal inspection of the visible parts of the inside of the colon, does not exist. Experts agree that it is desirable to inspect all quadrants of the colon wall while the endoscope is gradually withdrawn. In this paper, we present a new computer-based method that constructs a Quadrant Coverage Histogram to determine the number of quadrants of the colon wall inspected during the withdrawal phase of colonoscopy. The proposed method is part of our novel computer-aided quality control system for colonoscopy intended for use in routine clinical practice.