A prospective study was conducted on 20 consecutive patients who underwent elective exploration of common bile-ducts for stones to determine the optimal irrigation pressures in choledochoscopy. Ten patients had rigid choledochoscopy and 10 patients had flexible choledochoscopy. Choledochoscopic views were assessed at low irrigation pressure (irrigant delivered at hydrostatic pressure of 1 m of water or 80 mmHg) and high irrigation pressure (irrigant delivered at cuff pressure of 300 mmHg plus hydrostatic pressure of 80 mmHg). Choledochoscopic views were better with high than low irrigation pressures for rigid choledochoscopy. However, there was little difference in the views for flexible choledochoscopy. Common bile-duct pressures were measured with irrigant delivered at pressures 80-380 mmHg at intervals of 50 mmHg. With increasing irrigation pressures, the common duct pressures rose and they were consistently higher in flexible than rigid choledochoscopy. The common duct pressures in rigid choledochoscopy never exceeded the recommended safety limit, even at the high irrigation pressure of 380 mmHg. With flexible choledochoscopy, the common duct pressure was below the recommended safety limit only with low irrigation pressure of 80 mmHg. Therefore, it is recommended that a high irrigation pressure of 380 mmHg for rigid choledochoscopy and a low irrigation pressure of 80 mmHg for flexible choledochoscopy be used.