Hakim's hypothesis, which explains the brain's adaptation to pressure in hydrocephalus and the enlargement of ventricular volume, was studied with computed tomographic (CT) scanning. The critical element in the theory is that brain water decreases when there is a pressure gradient within brain tissue. Extraaxial hematomas uniformly increase the CT density of adjacent brain tissue, and this is best explained by water shifts out of the tissue. In hydrocephalus cases studied early after shunting (10 days or less), a decrease in ventricular volume is accompanied by an overall decrease in the CT density of the brain. These CT changes support Hakim's view of the brain as a sponge that can change its hydration under pathological conditions.