The dimensions of vessels forming the circle of Willis were studied in 100 fixed brains from cadavers. The coefficient of variation for length was greatest for the anterior communicating artery, and that for diameter was greatest for the posterior communicating artery. Abnormally narrow diameter occurred in 25 vessels of 24 circles and was most frequently seen in the posterior cerebral (11) and posterior communicating arteries (10). A significant inverse relationship existed between the diameters of the posterior cerebral and posterior communicating arteries of the same side. The left cerebral hemisphere appeared to enjoy a better blood supply.