Chiari type I malformation has traditionally been defined as a downward herniation of the cerebellar tonsils of ≥5 mm through the foramen magnum and it is likely associated with a volumetrically reduced posterior fossa. Syringomyelia is commonly associated with Chiari type I malformation. We estimate the prevalence of these two conditions and determine that they are more common than previously expected. We identify the genetic syndromes associated with some cases of Chiari type I malformation, and we provide evidence in favor of a genetic hypothesis for at least a subset of the nonsyndromic cases.