THE RETROSIGMOID TRANSMEATAL technique remains the approach of choice for hearing preservation during the removal of acoustic neuromas that protrude from the porus acusticus. However, encroachment into the bony labyrinth in an effort to remove the tumor in the lateral end of the internal auditory canal (IAC) continues to compromise hearing in certain cases. The limits in the safe removal of the posterior wall of the IAC are not generally agreed on. To address this problem, we have performed a morphometric analysis of 32 fixed cadaveric temporal bones by microsurgical dissection and measurement with fine-cut bone window computed tomographic (CT) scans. The morphometric relationships of identifiable surface landmarks were first determined. Fine cut bone window CT scans were next performed on each bone and the distances between the fundus, the vestibule, and the common crus (CC) with the internal auditory meatus (IAM) were determined. Additionally, the thickness of the bone overlying the posterior semicircular canal at the CC was measured. From a retrosigmoid trajectory, employing a 4-cm craniotomy, the posterior wall of the IAC was removed with a high-speed drill, limiting removal to the distance from the vestibule to the IAM, as determined by CT measurement. Preservation of the integrity of deep structures was confirmed by inspection. The length of the actual IAC unroofed was measured and was compared with the IAC length, from IAM to fundus, measured by CT. The average canal length by CT measured 10.0 mm ± 1.8 (range, 6.6–14.0). The length of the canal uncovered averaged 5.9 mm ± 1.4 (4.0–8.5). The vestibule, CC, and posterior semicircular canal were then uncovered, and measurements were taken to verify those determined by CT. The average distances from the IAM to the vestibule and CC closely correlated with those determined by CT and were 9.52 mm ± 1.93 (6.5–13.5) and 10.43 mm ± 2.13 (7.0–14.0), respectively. The average greatest thickness of bone overlying the posterior semicircular canal was 3.64 mm ± 1.43 (1.5–6.5). Analysis of all morphometric data failed to reveal a reliable correlate between surface landmark morphometry and the structures of the inner ear. From this study, we conclude the following: 1) there is significant variability in the morphometry of the adult temporal bone; 2) surface morphometric relationships may be generally unreliable as a guide to internal temporal bone anatomical relationships; 3) it is not safe in most cases to broadly apply a generalized guideline measurement for the unroofing of the posterior IAC without a high risk of violating the bony labyrinth or vestibule; 4) from a retrosigmoid trajectory, the extent of IAC unroofed is less than the measured length of the canal because of the angle of approach; 5) the length of the IAC that may be uncovered safely is, in general, proportional to the angle of approach; and 6) preoperative imaging with fine-cut bone window CT will provide a reliable and valuable adjunct to maximize the exposure of the IAC contents, while helping to minimize morbidity.