Hypothesis: Surgical operative performance benefits from analysis of the mechanisms underlying technical errors committed during surgery. Design: Prospective study using the Observational Clinical Human Reliability Assessment (OCHRA) system and complete unedited videotapes of the operations. Setting: Three National Health Service hospitals within the United Kingdom. Patients: Two hundred consecutive patients with symptomatic gallstone disease. Interventions: Elective laparoscopic cholecystectomy for symptomatic gallstone disease by surgeons, who were blind to the nature and objectives of the study, using their usual operative technique. Main Outcome Measures: Surgical consequential and inconsequential operative errors. Results: The analysis of 38062 steps of the 200 laparoscopic cholecystectomies performed by 26 surgeons identified 2242 errors. The mean±SD total, inconsequential, and consequential errors per surgical procedure were 11.0±8.0, 8.0±6.0, and 4.0±3.0, respectively. Dissection of the Calot triangle (second task zone of the operation) incurred more total errors (6.5±5.4) compared with the first (2.9±2.8, P<.001) and third (5.1±3.9, P<.05) task zones. This translated to a higher error probability (6.9% vs 3.5% for the first and 5.5% for third task zones). The combined sharp and blunt dissection method had fewer errors than the blunt/teasing dissection technique (9.45±7.6 vs 13.9±7.3, P<.001) although different surgeons were involved. The most serious consequences were encountered during dissection with the electrosurgical hook knife. Conclusion: This study has confirmed that the Observational Clinical Human Reliability Assessment system provides a comprehensive objective assessment of the quality of surgical operative performance by documenting the errors, the stage of the operation in which errors are enacted most frequently, and where these errors have serious consequences (hazard zones).