The anatomy of the caudal (sacral extradural) space was studied in 41 children, using magnetic resonance imaging. The distance from the upper margin of the sacrococcygeal membrane to the dural sac, the length of the membrane and the maximum depth of the caudal space were each measured. Age, height, weight and body surface area were recorded and, using multiple linear regression (stepwise technique), equations predicting the length of the membrane and the distance between its upper margin and the lower limit of the dural sac were obtained. Wide variability limits the clinical usefulness of these equations. In all patients, the maximum depth of the caudal space was found to be at the upper margin of the sacrococcygeal membrane. No correlation was found between this maximum depth and the age, height, weight or body surface area of the child.