The records of 3011 obstetric extradural blocks were examined to determine the depth of the extradural space and the relative risk of dural puncture. The overall incidence of dural puncture was 0.7%. A disproportionate number of punctures occurred when the space was superficial (1.45%), with a risk factor three times greater than that associated with normal depth spaces (0.48%). Caution is advised when infiltrating with local anaesthetics before extradural procedures, and when initially introducing the extradural needle.