The effect of needle size on postural headache after diagnostic lumbar puncture was studied in 300 patients at the Outpatient Department of Neurology, University of Helsinki. In three groups of 100 patients a needle size of 0.9mm (20 gauge), 0.7mm (22 gauge) and 0.6mm (23 gauge) were used, respectively. The total number of days with incapacitating postural headache in the groups diminished significantly with the decreasing needle size from 282 through 181 to 76 days. An analysis of CT scans performed in 95 patients showed that the incidence of post-lumbar puncture headache significantly decreased with brain atrophy seen on CT independently of the needle size. The results suggest that the thinnest needle (0.6mm) usable without special techniques should be chosen for diagnostic lumbar punctures at outpatient departments.