The common bile duct (CBD) diameter is one factor that clinicians use when deciding on invasive evaluation for intra-ductal pathology, e.g., endoscopic retrograde cholangiopancreatography. Previous studies and gastrointestinal and radiological textbook authors report disparate interpretations. These inconsistent interpretations likely result from methodological limitations in prior studies. The purpose of this work is to primarily compare the CBD diameter among patients with and without prior cholecystectomy and secondarily to compare proximal and distal CBD measurements. Among 40 matched pairs, post-cholecystectomy patients had larger mean CBD diameters at proximal (7.0 vs. 5.4 mm; P < 0.001) and distal (5.9 vs. 4.6 mm; P < 0.001) sites. Post-cholecystectomy patients were also more likely to exceed the 6-mm cut point for proximal (80 vs. 28%; P < 0.001) or distal (58 vs. 20%; P = 0.003) measurements. Incidental radiographic detection of enlarged CBDs among post-cholecystectomy patients is common; therefore, clinicians should use clinical determinants to guide decisions about additional costly or potentially harmful evaluation for intraductal pathology.