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To establish normal reference values for the presence of the anatomic variants of the circle of Willis and average diameters for its component vessels by using three-dimensional time-of-flight magnetic resonance (MR) angiography and to determine whether age- or sex-related differences exist in the circle's anatomy. One hundred fifty volunteers were grouped according to age: those aged 20-25 years (n = 50) and those aged 60-88 years (n = 100). All subjects underwent three-dimensional time-of-flight MR angiography of the arterial circle at 1.5 T. The anatomic variants of the anterior and posterior parts of the circle were determined separately, the completeness of the entire circle was assessed, and the diameters of all component vessels were measured. On MR angiograms, 111 (74%) subjects demonstrated a complete anterior part of the circle, 78 (52 …