Primary lateral sclerosis denotes a clinical state with the insidious onset and slow progression of neurological dysfunction limited to the corticospinal tract. We describe four unrelated patients with spastic paraparesis and normal sensory and cerebellar function. None had a family history of neurological disease. All had been symptomatic for more than five years and had been followed up for 25 to 42 months manifesting signs of involvement of other systems. Laboratory and roentgenographic evaluations demonstrated no contributory abnormality. Motor and sensory nerve conduction studies and electromyography disclosed no abnormality. These negative findings support the position that primary lateral sclerosis is a distinct neurological entity.