One of the responses of the skull to diminution in content or pressure is thickening of the cranium. In a few cases new bone slowly laid down along the inner table of the skull, after shunting for severe childhood hydrocephalus, diminishes internal calvarial dimensions and capacity appreciably. The inner table actually grows inward toward the retreating brain and intradural contents. This inward growth of the inner table may proceed in spite of stability or slow increase in outer table dimensions and head circumference.Measurement of outer table diameters and head circumferences in such cases fails to reflect internal volume changes.The sella sometimes becomes quite small.The probable physiologic basis of this shrinkage of calvarial capacity is presented briefly.