Lumbar puncture was performed in 195 children and the depth of needle was recorded. Our results show that the depth of lumbar puncture necessary to obtain uncontaminated cerebrospinal fluid correlates best with the child's weight. The simple formula: mean depth of insertion (cm) = 1.3 + 0.07 x body weight (kg), can be used to estimate the depth of lumbar puncture of children older than 3 months. The depths of lumbar puncture of children younger than 3 months are mostly 1.0-1.5 cm.