We measured changes in dural thickness to estimate intracranial pressure. The dural thickness on magnetic resonance imaging with contrast enhancement was compared in a hydrocephalic patient before and after shunt operation. Dural thickness also was measured directly using a micrometer at craniotomy for aneurysmal clipping in 11 patients. A small ultrasound probe (5 MHz) was held against the temporal scalp of 10 volunteers to extract convoluted interference echoes from the dura mater using a computer--based system for fast Fourier transform-Cepstrum analysis and maximum entropy analysis. The degree of intracranial pressure in the supine position was varied in the volunteers with transient neck compression. The enhanced dural thickness of the patient with hydrocephalus, barely visualized before shunt operation, increased after surgery. Dural thickness measurements obtained ultrasonographically in the supine position were similar to direct measurements of thickness. Changes in dural thickness on ultrasonography reflect changes in intracranial pressure.