We sought to measure cystic duct diameter in patients without biliary calculi and in those with cholelithiasis or choledocholithiasis. Using endoscopic retrograde cholangiopancreatography (ERCP), we visualized the cystic duct in 168 patients referred to our unit. These patients were distributed into three groups based on findings at ERCP: Group I (N = 57), no calculi in the gallbladder or common bile duct; group II (N = 27), stones found in the gallbladder but absent from the common bile duct; and group III (N = 34), stones present in the common bile duct with or without gallbladder stones. The diameter of the cystic duct was measured at its widest and narrowest dimensions. The largest diameter measured was greater in group III (7.72 +/- 2.29 mm) than in groups I (2.63 +/- 0.67 mm) and II (4.59 +/- 1.13 mm) (P < 0.001). The same differences were found in measurement of the smallest diameter (5.00 +/- 0.99 mm, 3.10 +/- 0.62 mm, and 1.83 +/- 0.53 mm, for groups III, II, and I, respectively) (P < 0.001). Maximal and minimal cystic duct diameter show a progressive increase at each level of disease. This increase in cystic duct size may facilitate the migration of gallstone fragments after lithotripsy and facilitate the instrumentation of the cystic duct during ERCP and laparoscopic cholecystectomy.