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Objective. Transabdominal ultrasound (US) is the most frequently used imaging method for the diagnosis of choledocholithiasis. The aim of this prospective study was to evaluate the diagnostic accuracy of high-resolution US in the diagnosis of common bile duct stones depending on the operator's experience and in comparison with endoscopic retrograde cholangiography (ERC) as the gold standard. Material and methods. From April 2003 through November 2004, 126 patients referred because of clinically and biochemically suspected common bile duct stones were included in the study. Two patients were excluded because they refused to undergo ERC. Consequently, the study comprised 124 patients (86 F, 38 M, mean age 63.2 years, range 21–91 years). High-resolution US was performed (2–5 MHz sector scanner; Siemens Elegra, Erlangen, Germany) by operators who were unaware of the results of other imaging procedures. The definitive diagnosis was established by means of ERC. Results. Thirty-five out of 124 patients were investigated by experienced examiners. Twenty-seven of 35 patients (77%) were found to have stones at ERC. Bile duct stones were correctly found by US in 22 out of 27 patients (sensitivity 82%, 95% CI: 63–92). Of the 8 patients without stones at ERC, one false-positive diagnosis was made with US (specificity 88%, 95% CI: 53–98). Correct diagnoses were made in 29 out of 35 (accuracy 83%, 95% CI: 67–92) patients investigated by experienced examiners. Eighty-nine out of 124 patients were investigated by less-experienced examiners. Fifty-four of 89 patients (61%) were found to have stones at ERC. Choledocholithiasis was found correctly in only 25 out of 54 patients (sensitivity 46%, 95% CI: 34–59). Of the 35 patients without stones at ERC, three false-positive diagnoses were made with US (specificity 91%, 95% CI: 78–97). In conclusion, correct diagnoses were observed in 57 of 89 patients (accuracy 64%, 95% CI: 54–73) investigated by less-experienced examiners ( p [ABSTRACT FROM AUTHOR]