All surgical manipulations of the endolymphatic sac have a common requirement: to identify the intradural part. We studied the position of the intradural endolymphatic sac in relation to the posterior semicircular canal, the sigmoid sinus, the retrofacial air cell tract, and the jugular bulb in 50 temporal bones. The intradural sac was identifiable in 48 cases; in two the sac either was absent or would not have been identifiable surgically without jeopardizing other important structures, particularly the posterior semicircular canal.