The conventional approach to the fourth ventricle is by splitting the vermis on the suboccipital surface of the cerebellum. By a unilateral transcerebellomedullary fissure approach, it is possible to provide sufficient operative space from cerebral aqueduct to obex without splitting the vermis. This approach needs meticulous dissection of the cerebellomedullary fissure and preservation of the posterior inferior cerebellar artery (PICA) and its branches. The tonsillo-medullary and telovelotonsillar segments of the PICA are the most important vessels encountered in the transcerebellomedullary fissure approach. The PICA was examined under a surgical microscope in a total of 40 specimens by perfusing with a mixture of 10% Indian ink and gelatin. The passing of the tonsillomedullary segment of the PICA through the cerebellomedullary fissure was observed superior to the tonsil in 5%, at the level of the upper pole of the tonsil in 17.5%, at the middle of the tonsil in 37.5% and at the level of the lower pole of the tonsil in 37.5% specimens. When the PICA arose from the lateral medullary (LM) segment of the vertebral artery (VA), a caudal loop was present in 90%, when the PICA originated from the premedullary segment of the VA, the loop was present in 87.5% specimens. When the PICA arose from the basilar artery (BA), the loop was absent, and the tonsillomedullary segment of the PICA showed a straight course (100%). A thorough understanding of the relationship of the branches of the PICA to the cerebellar tonsils are prerequisites for surgery in and around the fourth ventricle.