An ELISA study has shown that plasma levels of antibodies against neurofilament (NF) proteins correlate with disease stage in amyotrophic lateral sclerosis (ALS). Elevated NF antibody levels were observed in patients with ALS compared with healthy controls. In addition, patients with advanced ALS exhibited higher levels of NF antibodies than did individuals at earlier stages of the disease. The results raise the possibility that NF immunoreactivity could be a biomarker for disease progression in ALS.