Eighteen autopsy cases of syringomyelia were studied neuropathologically. In 5 cases associated with Chiari type I malformation, the syrinx was irregular in shape and communicated with the subarachnoid space at the entry zone of the posterior nerve roots. The central canal above the level of the syrinx was patent in 2 but closed in 3 out of 5 cases. In cases associated with Chiari type I malformation, communication between the syrinx and the subarachnoid space was considered to play an important role in the pathogenesis of syringomyelia. In 6 cases associated with Chiari type II malformation, the syrinx central canal, and the central canal was patent from the 4th ventricle to the syrinx in all cases. In these cases, direct continuity between the 4th ventricle and the syrinx was essential for the development of the syrinx. In cases associated with posterior fossa or spinal canal tumors, the local circulatory disturbance and/or edema due to the tumors were thought to cause the syrinx.