We evaluated an automated polyp size measurement tool in computed tomographic colonography for its accuracy and value for patient risk stratification.A simulation program generated a raw data phantom with sessile and pedunculated polyps of known sizes using 120 to 140 kV and 50, 40, 20, 15, and 10 mAs. All polyps were measured by clicking on the polyp surface. Comparison of the calculated size with the known polyp sizes allowed calculation of reproducibility and accuracy. For patients with proven polyps, we also compared automated measurements with manual and endoscopic measurements to evaluate the effect on patient risk stratification.The automated measurement tool allowed accurate measurements. In the patient study, assignment to the correct size group was not significantly different from the radiologist's results. However, it slightly improved patient risk stratification by reducing both failed and unnecessary colonoscopy referral.An automated tool for polyp measurement in patients facilitates patient risk stratification.