The diameter of 261 middle cerebral and 225 anterior cerebral arteries was measured on half axial antero-posterior (ap) view, and the diameter of 217 internal carotid arteries on side view angiograms. Mean diameter was significantly larger by 9.3%, 8.8% and 9.7% respectively in males than in females. In the females the vessels of the left hemisphere were wider but not significantly so than those of the right hemisphere, and the contrary was true for the males. In both sexes the side difference was reversed in the eighth decennium. There was no correlation of vessel diameter with age except for a statistically significant increase by 7.8% of right middle cerebral artery diameter in males from the fifth to the sixth decennium.