HYPOTHESIS: Magnetic resonance cholangiography (MRC) offers the potential for accurate, noninvasive detection of common bile duct stones (CBDSs) before cholecystectomy, and for a consequent reduction in the incidence of preoperative negative diagnoses associated with endoscopic retrograde cholangiography (ERC). DESIGN: Prospective cohort study: MRC results were correlated with ERC (high-risk patients) or intraoperative cholangiography (moderate-risk patients). SETTING: A university hospital providing primary, secondary, and tertiary care. PATIENTS: Seventy patients with suspected CBDSs scheduled to undergo elective cholecystectomy between April 15, 1997, and September 30, 1998. Forty patients were considered at high risk and 30 at moderate risk for CBDSs, according to results of liver function tests and sonograms of the upper abdomen. MAIN OUTCOME MEASURES: Confirmation or exclusion of CBDSs by MRC was assessed by a panel of radiologists who were unaware of the ERC results. Results of ERC and intraoperative cholangiography were analyzed by the investigating gastroenterologists or surgeon. RESULTS: Results of MRC were positive for CBDSs in 21 (52%) of 40 high-risk patients, a finding confirmed by preoperative ERC in 19 (90%) of 21 patients. Results of MRC were positive for CBDSs in 6 (20%) of 30 moderate-risk patients, all of which were confirmed by intraoperative cholangiography. Finally, CBDSs were present in 19 (48%) of 40 high-risk patients and 6 (20%) of 30 moderate-risk patients (P = .02). Overall sensitivity and specificity of MRC were 100% and 95.6%, respectively; the positive and negative predictive values were 92.6% and 100%, respectively. CONCLUSIONS: Magnetic resonance cholangiography is a reliable, noninvasive method for the detection or exclusion of CBDSs, and seems to reduce the frequency of negative diagnoses associated with ERC. Magnetic resonance cholangiography revealed no CBDSs in 19 (48%) of 40 patients at high risk for CBDSs. Thus, MRC-based diagnosis has the potential to reduce the number of invasive preoperative diagnostic procedures and their associated risks and overall health care costs.