OBJECTIVE Increased common bile duct (CBD) diameter has been attributed to aging and previous cholecystectomy. These relationships are, however, controversial and based mainly on old studies and methodologies. Our objective is to evaluate the relationship between age, cholecystectomy, and other clinical factors and CBD diameter, as measured by endoscopic ultrasound (EUS). METHODS We carried out a retrospective cohort study including patients who underwent EUS in our institution. Patients with an obstructing lesion of the bile ducts, previous sphincter manipulation, or insufficient data were excluded. CBD diameter was measured as a routine part of the examination, in the most distal extrapancreatic portion, between its two exterior margins. The patients were divided into five age groups. The mean CBD diameter in each group was calculated and compared with the other groups. Effects of cholecystectomy, gender, time from operation, and elevated liver enzymes were also evaluated. RESULTS Six hundred forty-seven patients were included in the study (66% women). Twenty-three percent were postcholecystectomy. There was no difference between the first three groups regarding CBD diameter, but it was significantly wider in groups 4 and 5 (p < 0.001). In all age groups, the postcholecystectomy patients had significantly wider CBD than those with an intact gallbladder (in all groups, p < 0.01). CONCLUSIONS This EUS study confirms that the CBD dilates significantly after the age of 70 years, but even in the most elderly patients, with an intact gallbladder, the normal CBD does not exceed 7.6 mm, thus a wider CBD warrants further investigation. The single additional factor contributing to dilatation of the CBD was cholecystectomy. A linear regression equation is proposed for the prediction of CBD diameter.