Primary lateral sclerosis (PLS) has been defined as a rare. Non-hereditary disease characterized by progressive spinobulbar spasticity, related to the exclusive involvement of precentral pyramidal neurons, with secondary pyramidal tract degeneration and a preservation of anterior horn motor neurons, the latter allowing PLS to be distinguish from amyotrophic lateral sclerosis (ALS). However, a clear distinction between the two diseases remains a subject of debate. With this in mind, we assessed patients with meeting the previously published criteria for PLS in a prospective, longitudinal study. At regular intervals, we analyzed various clinical and electrophysiological parameters in nine patients with a diagnosis of PLS. We made a deltoid muscle biopsy and PET study.Our results provide evidence that degeneration in PLS is not restricted to the upper motor neurons but also affects the lower motor neurons. The distinction between ALS and PLS is related to the degree and stability of lower motor neuron involvement. In view of the similarities with ALS, we consider that PLS may represent a slowly progressive syndrome closely related to this disease.