Adults with slowly progressive noninherited gait disorders may show no abnormalities on examination other than signs implicating the corticospinal tracts. That is the syndrome of "primary lateral sclerosis" (PLS), a clinical diagnosis that has been avoided because it is a diagnosis of exclusion, proven only at autopsy. Now, modern technology can exclude other disorders that can cause the syndrome with an accuracy of about 95%. That serves to eliminate the following: compressive lesions at the foramen magnum or cervical spinal cord, multiple sclerosis, amyotrophic lateral sclerosis, Chiari malformation, syringomyelia, biochemical abnormality, and persistent infection with human immunodeficiency virus or human T-lymphotrophic virus type I. We studied three autopsy-proved cases of PLS; six living patients in whom PLS was diagnosed clinically after comprehensive evaluations that excluded the alternative diagnoses; and two patients with this syndrome of PLS and antibodies to human immunodeficiency virus seropositivity that clinically resembled PLS. Primary lateral sclerosis is now a respectable and permissible diagnosis.