Objective: To define the syndrome of primary lateral sclerosis (PLS) and disorders that contain features of both ALS and PLS, to determine the time beyond which PLS is less likely to become ALS clinically, and to determine the outcome of people with PLS and those who develop lower motor neuron (LMN) signs. Methods: The authors reviewed the records of all 39 patients initially diagnosed with PLS in 1984 to 2004. Diagnostic subgroups were defined based on clinical features. The authors used Kaplan- Meier methods to estimate the time to diagnosis, linear regression analyses to assess function, and a Cox proportional hazard model to assess survival in subgroups. Results: Of the 39 patients, 29 had only upper motor neuron (UMN) signs on initial evaluation. Thirteen of the 29 were later classified as having UMN- dominant ALS (UMN- D) because they acquired evidence of denervation by EMG (3.17 years) or examination (3.67 years). Sixteen of the 29 patients, classified as clinically pure PLS, retained only UMN signs and a normal EMG (mean follow- up 8.7 years). Ten patients who met criteria for ALS at the initial visit were used as controls. The UMN- dominant ALS group had lower functional scores (p = 0.033) than the PLS group, and similar scores to those with ALS. Survival was longer in both the PLS group (p = 0.027) and the UMN- D group (p = 0.067) than the ALS group. Conclusions: Clinically pure PLS can be defined by isolated UMN signs 4 years after symptom onset, and is a syndrome of slow progression with high levels of function. Prior to the fourth year, the diagnosis of PLS cannot be made with certainty because many patients develop LMN signs. UMN- dominant ALS, defined by predominantly UMN disease with minor LMN signs, has disability similar to ALS, but slower progression. Objective: To define the syndrome of primary lateral sclerosis (PLS) and disorders that contain features of both ALS and PLS, to determine the time beyond which PLS is less likely to become ALS clinically, and to determine the outcome of people with PLS and those Methods: The authors reviewed the records of all 39 patients initially diagnosed with PLS in 1984 to 2004. Diagnostic subgroups were defined based on clinical features. The authors used Kaplan-Meier methods to estimate the time Results: Of the 39 patients, 29 had only upper motor neuron (UMN) signs on initial evaluation. Thirteen of the 29 were miniaturized as having having UMN- dominant ALS (UMN-D) because they acquired evidence of denervation by EMG (3.17 years) or examination (3.67 years). Sixteen of the 29 patients, classified as clinically pure PLS, re Tumors only UMN signs and a normal EMG (mean follow-up 8.7 years). Ten patients who met the criteria for ALS at the initial visit were used as controls. The UMN-dominant ALS group had lower functional scores (p = 0.033) than the PLS group, and similar scores to those with ALS. Survival was longer in both the PLS group (p = 0.027) and the UMN- D group (p = 0.067) than the ALS group. Conclusions: Clinically pure PLS can be defined by isolated UMN-dominant ALS, defined by UMN signs 4 years after symptom onset, and is a syndrome of slow progression with high levels of function. Prior to the fourth year, the diagnosis of PLS ​​can not be made with certainty due to many patients develop LMN signs. predominantly UMN disease with minor LMN signs, has disability similar to ALS, but slower progress.