Data from the literature about plasma and CSF amino acid (AA) levels in amyotrophic lateral sclerosis (ALS) remain controversial. To refine such analyses we used HPLC, and report a study of plasma and CSF AA concentrations in patients with ALS, the type of the disease (spinal and bulbar onset) being precisely determined. In ALS, there is a decrease in the plasma levels of the large neutral amino acids (LNAA) alanine, isoleucine, leucine, methionine and tyrosine which was particularly striking in the bulbar type (p < 0.05). Plasma glutamate levels do not differ between ALS and controls, but are significantly increased in ALS with spinal onset and decreased in the bulbar type (p < 0.05 vs controls, p < 0.001 bulbar vs spinal). In CSF, the analysis of the whole ALS group shows no difference from controls. However, there is an increase of CSF serine, glutamine and alanine in ALS with spinal onset (p < 0.05). Our results do not support an abnormal profile of excitatory AA concentrations in ALS. The heterogeneous changes we observed, mainly concerning LNAAs, may be explained by a blood-CSF barrier disturbance in the disease. As AA levels clearly differ between ALS types, with low concentrations in bulbar ALS, this dual profile probably explains some of the discrepancies between previous studies.