The invention provides a method for diagnosing amyotrophic lateral sclerosis (ALS) in a subject, a method for assessing the effectiveness of a drug in treating ALS, and a method for determining the site of onset of ALS in a subject. Each method comprises (a) obtaining a sample from the subject, (b) analyzing the proteins in the sample by mass spectroscopy, and (c) determining a mass spectral profile for the sample. In some embodiments, the method comprises comparing the mass spectral profile of the sample to the mass spectral profile of a positive or a negative standard.