The optic nerve, ontogenetically part of the central nervous system, is surrounded by subarachnoidal cerebrospinal fluid (CSF) and dura mater. Because of the connection with the intracranial subarachnoidal space, CSF pressure variations influence the optic nerve sheath (ONS) diameter. Histologic studies revealed a segment of the optic nerve in which maximal diameter fluctuations could be expected, namely the bulging dura mater region approximately 3 mm behind the papilla. Twenty preparations of optic nerves obtained post mortem were examined sonographically before and after dilatation of the ONS, by means of measurement from three different projections. After gelatine-induced widening of the subarachnoidal space, the mean diameter increased by 60% at 3 mm behind the optic nerve head, but only by 35% at 10 mm distance. Independent measurements by two examiners correlated highly, which indicates excellent reproducibility of the sonographic measurements. The optimal experimental scanning position was at a right angle to the optic nerve (longitudinal section). Under clinical conditions, however, only axial sections can be obtained using anterior probe positions with transbulbar sound directions. Using such axial projections the 3 mm position proved reliably reproducible. The reduced resolution of the optic nerve itself, allowing it to be distinguished from its surrounding sheath, proved to be somewhat disadvantageous from this projection angle.