The hereditary spastic paraplegias (HSPs) are rapidly being understood as a large group of genetically heterogeneous disorders in which the predominant clinical feature is gait disturbance due to lower-extremity spasticity and weakness. The degree of severity and manner of progression are often quite variable between different genetic types of HSP, between different families with the same genetic type of HSP, and between affected subjects from the same family who share exactly the same HSP gene mutation. Presently, genetic loci (designated SPG1 through SPG23 in order of their discovery) have been identified for 10 autosomal dominant, 8 autosomal recessive, and 3 linked types of HSP (designations SPG18 and SPG22 have been reserved for as yet unpublished loci).1,2 Ten HSP genes have been discovered.1,3