Primary lateral sclerosis (PLS) is a rare degenerative disorder of the upper motor neuron. Its nosological status and relationship to other motor neuron syndromes, especially amyotrophic lateral sclerosis (ALS), is uncertain. Diagnostic criteria have been proposed. We discuss the history of this rare clinical disorder, its relationship to the motor neuron disease syndrome, and reports of overlapping clinico-pathological conditions. Two patients with the clinical syndrome of PLS are described to illustrate current understanding of the clinical, laboratory, and neurophysiological features.