INTRODUCTION: The aim of the study was to determine if the common bile duct (CBD) diameter increases physiologically with age and investigate whether gallstones lead to changes in CBD diameter. METHOD: Our study is composed of 721 patients in 2 groups: 517 asymptomatic patients and 204 gallstones patients who underwent MRCP. Their CBD diameter was measured at its widest visible portion on MRCP. Simple linear regression of the average of these measurements was used to investigate the relationship between age and CBD diameter in both populations. Subjects were further divided into 8 subgroups age-wise, respectively, and their means compared decade-wise using ANOVA. Furthermore, each subgroup mean from both populations was compared with each other using a t test. RESULTS: Among the 517 subjects, the mean CBD diameter was 5.3663 mm ± 1.43546 and correlated with age (p < 0.01), dilating at 0.07 mm/year. The mean diameter of our oldest group was 7.9 mm, resulting in a plausible upper limit of 8 mm for the asymptomatic population. The mean CBD diameter of the cholelithiasis population was 5.6738 mm ± 1.40986 and also correlated with age (p < 0.01). The mean CBD diameter of the age groups from the cholelithiasis population was larger than that from the asymptomatic population, but none were statistically significant. CONCLUSION: Although the CBD increases physiologically with age, there is no significant difference in CBD size between the general population and the cholelithiasis patients at any particular age. Gallstones do not independently dilate the CBD.