In this single-blind, randomized study of post-lumbar-puncture headache (PPH) in 300 neurologic inpatients the significance of body posture after lumbar puncture (LP) was evaluated. Immediate mobilization was compared with bed rest for 6 h (3 h prone followed by 3 h supine posture). Contrary to the widely held belief, this investigation did not show significant differences between recumbent and ambulant patients as to frequency of PPH in the total material (39% versus 35%) or when men (31% versus 29%) and women (48% versus 41%) were evaluated separately. Headache associated with nausea was significantly more frequent in the recumbent than in the ambulant patients both in the total material (23% versus 13%) and in women (35% versus 16%). Thus, immediate mobilization seems to be preferable after LP.