The aim was to establish a reference range of measurements for all major Circle of Willis (COW) arteries for an Australian population of patients presenting for brain magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) that is typical of a tertiary referral hospital; and to report the prevalence and disease associations of COW variants in our patient population. All technically diagnostic MRI and MRA studies performed at our magnetic resonance (MR) centre in the calendar year 2006 were re-read by one reader who classified the COW anatomical layout and measured diameters of all the major vessels at defined locations. A subset of 30 was independently re-read by another reader and the interobserver measurement variability analyzed. Graphical analysis, with 95% confidence intervals (95CI), summary statistics reporting, t-testing for unpaired and for paired means, Hauck-Anderson (H-A) clinical equivalence testing and logistic regression analysis for categorical variables was performed as relevant. One hundred and seventy-one studies (of 187 eligible) formed the analysis population. All of our patients had vessels of comparable calibre regardless of MRI disease status, gender or COW variant (P > 0.05). Basilar artery (BA) diameter showed significant association with age, but other vessel diameters did not. The reference range is widely applicable, methodology straightforward and appears tolerant of interobserver variability. A number of discontinuous COW variants become more prevalent with age, perhaps from atherosclerotic occlusive disease. There was no association between COW variants, gender, aneurysm location or MRI disease status.