Methotrexate-induced neurotoxicity (MTX-Ntox) is a frequent complication of methotrexate (MTX) therapy for patients with both malignant and inflammatory diseases. MTX-Ntox can occur after intrathecal MTX or after low-, intermediate-, or high-dose systemic administration. Symptoms can present in the acute, subacute, or late setting form, and can range from affective disorders, malaise, and headaches, to somnolence, focal neurologic deficits, and seizures. While the pathogenesis of MTX-Ntox is likely multifactorial, one potential biochemical pathway leading from MTX to neurotoxicity involves the folate dependent remethylation of homocysteine (Hcy). MTX therapy is known to cause elevations of both plasma and CSF Hcy. Hcy is directly toxic to vascular endothelium and it and its metabolites are excitatory agonists of the N-methyl-D-aspartate (NMDA) receptor. Competitive or noncompetitive antagonists might afford protection from or reversal of MTX-Ntox. Using high-performance liquid chromatography (HPLC) with coulometric electrochemical detection, the authors measured CSF Hcy in sequential patients with severe subacute MTX-Ntox. CSF Hcy was higher in these patients (n = 9, median = 0.93 microM) than in asymptomatic patients (n = 11, median 0.2 microM, p < .01). Five patients with severe subacute MTX-Ntox (most with dysarthria and/or hemiplegia) were treated with 1-2 mg/kg oral dextromethorphan (DM), a noncompetitive antagonist of the N-methyl-1-aspartate (NMDA) receptor. All five had resolution of symptoms. These data provide additional clinical support for elevated CSF Hcy in the induction of MTX-Ntox through activation of the NMDA-receptor. These data provide support for a placebo-controlled clinical trial to examine the ability of DM to prevent or alleviate MTX-Ntox.