Summary:The pathogenesis of syringomyelia of Arnold-Chiari malformation (ACM) has been debated for decades. Theories abound; each fits some cases, but none fits all. Recently, it seems clear that the herniation of the cerebellar tonsils into the foramen magnum is the most constant feature of ACM. The herniated cerebellar tonsils at the foramen magnum may induce three events: (a) a check-valve effect at the central canal of the spinal cord, (b) a functional occlusion of the subarachnoid space at the foramen magnum, and (c) a functional occlusion of the foramen of Magendie due to mass effect, scarring arachnoiditis, or both. Mathematically, eight pathological possibilities may spring from the three events. An analysis of the eight pathological possibilities comes to the conclusion that a block of the subarachnoid space at the foramen magnum is a prerequisite for the pathogenesis of syringomyelia. The hydrodynamic theory of Gardner is applicable to one of the conditions where the outlet of the fourth ventricle is inadequate and the check-valve effect at the central canal of the spinal cord is absent. The therapeutic implications from such a conclusion are discussed.