Patients with a diagnosis of PLS per criteria by Pringle et al1 or with definite ALS per El Escorial criteria4 reviewed in the Motor Neuron Diseases Clinic at the University of Western Ontario from 1990 to 2006 were included in the analysis. All patients underwent electromyography/nerve condition studies (EMG/NCS). Primary lateral sclerosis was diagnosed in patients only if they had no evidence of acute or chronic denervation on EMG/NCS. We evaluated both symptoms and signs at the time of onset (ie, age, dysarthria, dysphagia, paresthesias, fasciculations, cramping, weakness, wasting, fatigability, stiffness, and dyspnea) in addition to symptoms and signs at follow-up (ie, bulbar dysfunction, wasting, pyramidal dysfunction, dementia, parkinsonism, and cerebellar, cortical, and sensory signs) for significant differences between the 2 patient populations. We also assessed the 2 groups for differences in comorbid diseases, including diabetes, epilepsy, cardiovascular disease, trauma, hypothyroidism, respiratory disease, rheumatological disease, malignancy, autoimmune disorder, infection, other neuromuscular disease, stroke/transient ischemic attack, human immunodeficiency virus (HIV) risk, and smoking status. This information was obtained during the initial visit and thus reflects premorbid conditions. Mann-Whitney tests were used to look for significant differences between the 2 groups. Two logistic regressions were run. The first included presenting symptoms and age at onset, and the second included follow-up symptoms and age at onset.