There is still a debate whether primary lateral sclerosis (PLS) is a distinct pathological entity or whether it represents one end of a continuous spectrum of motor neuron disease (MND). In this report we present four PLS patients who have been observed from the time of symptom onset (1990-1999) through January 2007. All of them have had only upper motor neuron (UMN) signs and slow clinical progression. Three patients have been presented with spastic paraparesis. Spasticity was the main clinical feature in demonstrated cases with hyperactive deep tendon reflexes, clonus, and Babinski signs. One patient was presented with spastic dysarthria at the disease onset. Mean disease duration, measured from symptom onset to the present, was 11.5 years in our reported series. All four PLS patients had not developed lower motor neuron (LMN) signs during this time of observation. This prospective analysis of our PLS series is in agreement with data from other studies suggesting that pure PLS cases have a prolonged course of disease with a high level of independence when compared to other MND.