BACKGROUND Magnetic resonance cholangiopancreatography (MRCP) is a popular modality for evaluation of the biliary tract, yet there is no data on the normal common bile duct (CBD) size of the average Taiwanese adult. This study attempts to establish a reference range for CBD diameter for the Taiwanese population. METHODS Over a 2-year period, all adults who underwent abdominal magnetic resonance imaging for health screening were recruited into the study. Patients with a prior history of hepatobiliary surgery and other significant morbidity were not included. Patients who were found to have abnormal liver function test results or abnormal imaging findings were also excluded from the study. After the patients fasted for a minimum of 8 hours, MR imaging was performed with a 1.5 T MR imager using a phased-array coil. Breath-hold thick slab single-shot turbo spin echo (ssTSE BH) projections were obtained, and these were used for CBD and portal vein diameter (PVD) measurement by workstation software. Initial recruitment included 265 patients, of which 66 were excluded due to abnormal liver function tests, seven more were excluded due to excess imaging artifacts or incomplete CBD visualization, and five were excluded due to other abnormal blood tests. This yielded a final study group of 187 patients between the ages of 21 and 78 years, which comprised 69 women and 118 men. RESULTS The mean CBD diameter is 4.6 mm, with a range from 1.76 to 10.49 mm. CBD diameters are significantly different in patients both younger and older than 65 years of age (p < 0.05), and are not significantly related to gender, serum glucose level, cholesterol level, hepatitis status and PVD. CONCLUSIONS Our study showed that the average CBD diameter for an asymptomatic Taiwanese adult is 4.6 mm, with an upper limit of 10.49 mm. CBD diameter is only significantly correlated with age. This is a useful reference in today's clinical setting where MRCP are commonly performed for evaluation of suspected biliary tract disease.