We studied the effect of CSF drainage on cerebral blood flow (CBF) in normal pressure hydrocephalus (NPH) and non-NPH dementia using the 133Xenon inhalation technique. Dementia patients had lower CBF than matched elderly normals. Flow values for NPH and non-NPH patients did not differ before or after CSF drainage. CBF did not increase after lumbar puncture, and these measurements were not useful in predicting the outcome of ventricular shunt surgery. Postoperative CBF did not increase after successful shunting.