We used transcranial Doppler ultrasonography to investigate whether a change in hemodynamics in the major arteries of the brain base occurred after diagnostic lumbar puncture. On the day before diagnostic lumbar puncture the flow in the right and left middle cerebral artery was measured in 36 patients using transcranial Doppler ultrasonography. 48 hours after lumbar puncture a second ultrasound examination was performed. We found that only patients with post-lumbar puncture headache (PLPS) showed a significant reduction in the flow of the right middle cerebral artery (p less than or equal to .05). These findings support the puncture-hole-seepage theory as pathogenetic principle of PLPS.