Familial occipitalization of the atlas with atlantalization of the axis was defined as a single congenital disease in Arabian horses following a clinical, radiologic, and morphologic study of 16 horses with congenital malformations of the occiput, atlas, and axis, and from a study of three reported cases. The constant morphologic features were interpreted as congenital atlantooccipital fusion, hypoplasia of the atlas and dens, malformation of the axis, and modification of the atlantoaxial joint. Atlantoaxial subluxation was also a frequent finding. The clinical syndromes shown by these horses were variable but were broadly classified into four groups. Neurologic signs varied from symmetric ataxia, with hypometria and tetraparesis to tetraplegia. These often progressive clinical signs were due to a progressive and focal compressive myelopathy of the proximal cervical spinal cord. Pedigree analysis demonstrated the familial nature of this occipitoatlantoaxial malformation seen in horses of only the Arabian breed.