Primary lateral sclerosis is a rare motor neuron degenerative disease of unknown aetiology. The disease occurs sporadically. Genetic mutations have been detected in only few cases. Symptom onset is usually in the fifth or sixth decade of life. Primary lateral sclerosis is a progressive disease involving upper motor neuron dysfunction. Typically, clinical symptoms begin in the lower extremities, but can also begin within the bulbar region, or other regions of the body. These clinical symptoms include spasticity, hyperreflexia, and weakness. No fasciculation, muscle wasting, or sensory symptoms have been observed. The disease is slowly progressive. Diagnostic criteria for this disease are based on the overall analysis of clinical symptoms as well as additional diagnostic tests. Diagnosis is based on the exclusion of other diseases. There is no specific marker for this disease. Also, no effective treatment has been reported. Some authors suggest that primary lateral sclerosis transforms into amyotrophic lateral sclerosis when clinical or electrophysiological signs of lower motor neuron damage appear. There are different points of view as to whether it represents a separate motor neuron disease, or a slowly progressing type of amyotrophic lateral sclerosis. The latest version of the El Escorial criteria classifies primary lateral sclerosis as a limited phenotype of amyotrophic lateral sclerosis.