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Background: Patients with good-grade subarachnoid hemorrhage (SAH) are those without initial neurological deficit. However, they can die or present severe deficit due to secondary insult leading to brain ischemia. After SAH, in a known context of energy crisis, vasospasm, hydrocephalus and intracranial hypertension contribute to unfavorable outcome. Lumbar puncture (LP) is sometimes performed in an attempt to reduce intracranial pressure (ICP) and release headaches. We hypothesize that in good-grade SAH patients, a 20-ml LP releases headaches, reduces ICP and improves cerebral blood flow (CBF) as measured with O15 PET scan. Methods: Six good-grade (WFNS grade 1or 2) SAH patients (mean age 48 years, 2 women, 4 men) were prospectively included. All aneurysms (4 anterior communicating artery and 2 right middle cerebral artery) were coiled at day 1. Patients were managed according to our local protocol. LP was performed for severe headache (VAS >7) despite maximal painkiller treatment. Patients were included when the LP was clinically needed. The 20-ml LP was done in the PET scan (mean delay between SAH and LP: 3.5 days). LP allows hydrostatic measurement of ICP. Arterial blood pressure (ABP) was noninvasively gauged with photoplethysmography. Every signal was monitored and analyzed off-line. Regional CBF (rCBF) was measured semiquantitatively with O15 PET before and after LP. Then we calculated the difference between baseline and post-LP condition for each area: positive value means augmentation of rCBF after the LP, negative value means reduction of rCBF. Individual descriptive analysis of CBF was first performed for each patient; then a statistical group analysis was done with SPM for all voxels using t statistics converted to Z scores (p 3.2). Results: A 20-ml LP yielded a reduction in pain (–4), a drop in ICP (24.3 ± 12.5 to 6.9 ± 4.7 mm Hg), but no change in ABP. Descriptive and statistical image analysis showed a heterogeneous and biphasic change in cerebral hemodynamics: rCBF was not kept constant and either augmented or decreased after the drop in ICP. Hence, cerebrovascular reactivity was spatially heterogeneous within the brain. rCBF seems to augment in the brain region roughly close to the bleed and to be reduced in the rest of the brain, with a rough plane of symmetry. Conclusions: In good-grade SAH, LP releases headaches and lowers ICP. LP and the drop in ICP have a heterogeneous and biphasic effect on rCBF, suggesting that cerebrovascular reactivity is not spatially homogeneous within the brain.