To ascertain the frequency and risk factors for post-lumbar puncture headache, we studied all adults who had an ambulatory lumbar puncture (LP) in a 1-year period. Patients filled out a questionnaire detailing their headache experience on the day of, and the 6 days following, LP. Those who did not return the questionnaire were telephoned. We systematically collected and analyzed many items, including the lumbar puncturist's experience, the degree of difficulty of the LP, CSF findings, final diagnosis, and the patient's demographic characteristics. Patients reporting headache before LP were more likely to report post-LP headaches. In addition to this, younger female patients with a lower body mass index have the highest risk of developing post-LP headaches. CSF opening pressure, cells, and protein, patient's position during LP, the duration of recumbency following LP, and the amount of CSF removed at the time of LP did not influence the occurrence of headache.