August Bier, the father of spinal anesthesia, suffered and reported the first lumbar puncture (LP) headache. On August 24, 1898 his assistant, a Dr. Hildebrandt, attempted to administer a spinal anesthetic to Dr. Bier; it was never completed because the syringe did not fit the already implanted spinal needle. Bier himself suggested that continued leakage of cerebrospinal fluid (CSF) through the dural puncture site was the cause of headache, a theory that has been embraced by the medical community; however, the mechanism is probably more complex. Nearly 50 years ago, J. Lawrence Pool, using an endoscopic technique to visualize the surface of the spinal cord and the cauda equina, frequently observed large collections of epidural fluid two to four days following lumbar puncture in patients without headache. Evidence that will be presented below suggests that CSF volume alterations may be the signal closest to the headache mechanism.