The certainty with which the L3-4 vertebral interspace can be identified was investigated by studying 50 cadavers. These were allocated randomly to two groups, differing in the way in which the L4 spinous process was identified. Identification of L4 by physically constructing Tuffier's line on the subject led to an increase in the number of catheters sited at the correct level (P less than 0.01).