THE EXTRADURAL MIDDLE fossa transpetrosal approach has been used to access lesions of the petroclival and posterior cavernous sinus regions by several neurosurgical groups, including our own. This is a technically demanding approach that provides a relatively wide extradural corridor interposed between the 5th cranial nerve and the cranial nerve VII-VIII complex, which minimizes brain retraction. We performed a microanatomical study to determine the limits of this exposure and in particular how the removal of the petrous bone could be maximized through this approach. Dissection of 15 fixed human cadaveric heads and 8 isolated temporal bones was performed to yield 38 sides studied. We identified a rhomboid-shaped construct of middle fossa landmarks that serve as a guide to maximally removing the petrous apex. The points defining this construct are as follows: 1) the junction of the greater superficial petrosal nerve and the trigeminal nerve; 2) the lateral edge of the porus trigeminus; 3) the intersection of the petrous ridge and arcuate eminence; and 4) the intersection of the lines extended along the axes of the greater superficial petrosal nerve and arcuate eminence. A morphometric analysis determined the average lengths of the respective sides of the complex to be 13.2 mm ± 2.6 × 22.2 mm ± 2.8 × 16.4 mm ± 3.4 × 16.6 mm ± 1.5 (beginning at “1” and proceeding sequentially), with an approximate area of the construct equal to 2.9 square centimeters. Maximal bone removal was found to extend inferiorly beyond the inferior petrosal sinus to the clivus, posteriorly to unroof the internal auditory canal completely, anteriorly under the trigeminal ganglion to include the petrous apex, and laterally to the geniculate ganglion and carotid artery. A simple geometric construct was also devised that proposes to assist in locating the cochlea.