In order to investigate mechanisms underlying the occurrence of bile duct injuries (BDIs) during laparoscopic cholecystectomy (LC), we analyzed results for 34 patients (0.59%; 17 men, 17 women; average age, 57 years) with BDI out of 5750 LCs, based on questionnaire responses from surgical operators, records of direct interviews with these operators, operative reports, and videotapes of the operations. The indications for LC in the 34 patients were chronic cholecystitis in 32 patients and acute cholecystitis in 2. The BDIs in these patients were divided into four classes using the Stewart-Way classification: class I, incision (incomplete transection) of the common bile duct (CBD), n = 6 (17.6%); class II, lateral damage to the common hepatic duct (CHD), n = 9 (26.5%); class III, transection of the CBD or CHD, n = 15 (44.1%); and class IV, right hepatic duct or right segmental hepatic duct injuries, n = 4 (11.8%). In all class III and 3 class I cases (18 in total; incidence 53%), the mistake involved misidentifying the CBD as the cystic duct. Of all types (classes) of injuries, class III injuries showed the mildest gallbladder inflammation, and there was a significant (P = 0.0005) difference in the severity of inflammation between class II and III injuries. We conclude that complete transection of the CBD, which is rare in laparotomy, was the most common BDI pattern occurring during LC and that the underlying factor in the operator making this error was mistaking the CBD for the cystic duct.