Primary lateral sclerosis and amyotrophic lateral sclerosis (ALS) are considered to be clinically distinct progressive paralytic neurodegenerative disorders. Following a period of diagnostic confusion, the clinical distinction between ALS and PLS became clear and diagnostic criteria established. PLS is characterized by degeneration of the upper motor neurons and the corticospinal and corticobulbar tracts, whereas ALS is a more severe disorder characterized by degeneration of both the upper and lower motor neurons.