Amyotrophic lateral sclerosis (ALS) is a late-onset, progressive motor neuronal degenerative disease occurring as sporadically and as a familial disorder. The patients with ALS typically become progressively paralyzed and develop respiratory failure that eventually leads to death within 3-5 years. For this disease, there is no effective diagnostic method and also drug. This report describes a simple and useful diagnostic biomarker for ALS. Our findings suggest that the combination analysis of a metabolite of prostaglandin D2, 11,15-dioxo-9-hydroxy-,2,3,45-tetranorprostan-1,20-dioic acid (tetranor PGDM and tPGDM) with creatinine is the diagnostic approach for ALS with high accuracy. tPGDM has the potential to be an important diagnostic tool in the pre-symptomatic stages and progression evaluation of ALS, and also to be a biomarker for the evaluation of drug effect. (C) 2011 Elsevier Inc. All rights reserved.