American College of Emergency Physicians (ACEP) [1] recommends that patients presenting with acute non-traumatic headache concerning for subarachnoid hemorrhage (SAH) undergo lumbar puncture (LP) when non-contrast head computed tomography (CT) is negative. The diagnostic yield of this approach is unknown. Evaluate the diagnostic yield, lengths of stay and complication rates of LPs in patients undergoing Emergency Department (ED) evaluation for aneurysmal SAH. Multi-center, retrospective, hypothesis-blinded, explicit chart review of patients undergoing ED-based lumbar puncture between 2007 and 2012. Charts of neurologically intact patients presenting with headache that had a negative head CT and underwent LP primarily to rule out SAH were reviewed. Trained data abstractors blinded to study hypothesis used standardized data forms with predefined terms for chart abstraction. We re-abstracted and assessed inter-rater agreement for 20 percent of charts with a 100 percent inter-rater agreement. Data were descriptive, using 95 percent confidence intervals. 1,282 LPs were performed, and 342 patients met inclusion criteria but only 1 percent were deemed positive for SAH in the chart. No aneurysm or vascular malformation was identified in those with positive LPs for SAH. Complications were in 4 percent and xanthochromia was found in 13 percent. Total length of stay was 7.8 hours (0.95 CI; 7.5 - 8.2). No patient discharged from the ED after a negative workup for SAH was re-admitted for SAH or underwent a neurosurgical procedure during a three-month follow-up period. LP in our cohort of neurologically intact CT-negative ED headache patients did not identify any cases of aneurysmal SAH but was associated with serious complications, a significant false positive rate, and extended ED length of stay.