A safe paramedian approach to the rhomboid fossa for surgical treatment of intrinsic brainstem lesions is based on detailed knowledge of the morphometric anatomy of superficially located motor structures. The morphometry of the rhomboid fossa is described in this report on the basis of histological studies conducted in six human brainstem specimens, with special emphasis on the colliculus facialis and the trigona nervi hypoglossi and vagi. Morphometric data include analysis of shrinkage factors in each specimen. The colliculus is a landmark for the nervus facialis, oculomotor nuclei, and the paramedian pontine reticular formation. In the surgeon's view from the posterior approach, the colliculus covers an area of 5.7 mm in the mediolateral and 6.8 mm in the craniocaudal direction and is located 0.6 mm lateral to the median sulcus. The fibers of the nervus facialis come as close as 0.2 mm to the surface of the fourth ventricle. The colliculus is located 15.7 mm above the obex. The trigona nervi hypoglossi and vagi cover a rectangular area measuring 3.1 by 6.5 mm and serve as a landmark for lower cranial nerve nuclei. These nuclei are located 0.3 mm lateral to the midline. An area with a maximum extension of 0.9 cm between the colliculus and trigona can be used for an infracollicular paramedian approach. The same applies to a supracollicular approach cranial to the colliculus and caudal to the fibers of the nervus trochlearis within the medullary velum, with a craniocaudal extension of 4 mm. Superficial motor nuclei and fibers can be identified by neurophysiological mapping, which helps to define safe surgical corridors into the rhomboid fossa, thus reducing functional morbidity caused by the operative approach in intrinsic pontine and pontomedullary lesions.