Ten patients with syringomyelia associated with Arnold-Chiari Type I malformations were evaluated. In each patient, a manometric Queckenstedt test was performed with the neck in various positions. No patient showed evidence of a block to the flow of cerebrospinal fluid (CSF) with the neck in the extended position; however, all showed a complete CSF block with the neck in a flexed position. Posterior fossa decompression with a C1-2 laminectomy was performed in nine cases, after which Queckenstedt test demonstrated free CSF communication in all nine with the neck in extension, in a neutral position, and in flexion. Postoperative magnetic resonance imaging showed shrinkage of the syrinx in the patients who underwent surgery. It is suggested that obstruction of the CSF pathway at the foramen magnum produced by neck movement is of importance in the formation and progression of a syrinx.