The post-lumbar puncture syndrome (PLPS) can best be explained by prolonged spinal fluid leakage owing to delayed closure of a dural defect. Its incidence after spinal anaesthesia is much lower than after diagnostic lumbar puncture (LP). This difference could be caused by a strand of arachnoid, which might enter the needle with the outflowing cerebrospinal fluid (CSF) during diagnostic LP and upon removal of the needle be threaded back through the dura to produce prolonged CSF leakage. To find a technique that further reduces the incidence of PLPS, this hypothesis was tested by evaluating the effect that reinserting the stylet before removing the needle had on the incidence of PLPS. By reinserting the stylet to the tip of the needle, the hypothesized strand would be pushed out, thereby reducing the frequency of PLPS. Sprotte's "atraumatic needle" (21 gauge) was used for LP. A total of 600 patients participated in the prospective study. They were randomized into two groups and questioned about their complaints every day for up to 7 days after the LP. All LPs were performed by two experienced neurologists (T.B., M.S.). In 300 patients, the stylet was reinserted to the tip of the eedle; in the other 300 it was not reinserted. Whereas 49 of the 300 patients without reinsertion developed PLPS, only 15 of the 300 patients with reinsertion did. This significant difference (16.3 vs 5.0%, P < 0.005, chi square test) supports our hypothesis. On the basis of our results, we recommend reinserting the stylet before removing the needle in order to reduce the incidence of PLPS.