The present invention provides a method of determining and treating amyotrophic lateral sclerosis (ALS) in a subject, comprising the following steps: (1) measuring a concentration of an autoantibody against high mobility group box 1 protein (HMGB1) in a biological sample from the subject and (2) administering an effective amount of ALS-treating drug to the subject whose measured concentration of the autoantibody against HMGB1 is higher than 0.874 μg/ml.
