OBJECTIVE The purpose of the study was to see the effect of age, sex, body mass index, previous cholecystectomy, hepatomegaly and fasting status on the common bile duct diameter. METHODS A series of 463 patients, 283 females and 180 males, with no hepatobiliary or pancreatic pathology were included in this study, the mean age was 45 +/- 16 years. Their age, sex, weight, height, fasting status and previous cholecystectomy was assessed and recorded by a physician prior to ultrasound examination. All patients were examined by real-time ultrasound to see if there was any pathology in the hepatobiliary and pancreatic area. Those with history of common bile duct exploration, endoscopic sphincterotomy or with previous history of cholecystectomy of less than 6 months and patients with common bile duct pathology were excluded from the study. The midportion of the common bile duct was taken as a fixed measurement for all patients and the size of the liver was also recorded. Analysis of variance as part of SPSS statistical package was used where common bile duct was considered a dependent variable, while sex, fasting status, hepatomegaly and previous cholecystectomy were considered to be independent variables, age and sex were considered as co-variants. RESULTS The factors found to be significantly affecting the diameter of the common bile duct (P<0.05) were age, previous cholecystectomy and body mass index. CONCLUSIONS If the CBD dilatation can not be explained by age, previous cholecystectomy and BMI, a pathology causing obstruction should be ruled out.