An episode of subacute encephalopathy after the infusion of a moderate dose of methotrexate (1500 mg/m2) (MTX) is reported in a young adult with metastastic gastric cancer. Weakness of the right arm, focal seizures, lethargy and confusion appeared on day 10. High signal intensity in periventricular white matter was observed on T2-weighted magnetic resonance imaging. Symptoms resolved spontaneously and completely after 48 h. We believe that this represents an unusual case of moderate-dose MTX-induced neurotoxicity in a patient with gastric cancer, which has not previously been reported.