A 61-year-old man was treated with combination chemotherapy incorporating cisplatinum, etoposide, high-dose 5-fluorouracil (2,250 mg/m2/24 hours) and folinic acid for an inoperable gastric adenocarcinoma. He developed acute neurologic symptoms of mental confusion, disorientation and irritability, and then lapsed into a deep coma, lasting for approximately 40 hours during the first dose (day 2) of 5-fluorouracil and folinic acid infusion. This complication reappeared on day 25 during the second dose of 5-fluorouracil and folinic acid, which were then the only drugs given. Because folinic acid was unlikely to be associated with this condition, neurotoxicity due to high-dose 5-fluorouracil was highly suspected. The pathogenesis of 5-fluorouracil neurotoxicity may be due to a Krebs cycle blockade by fluoroacetate and fluorocitrate, thiamine deficiency, or dihydrouracil dehydrogenase deficiency. High-dose 5-fluorouracil/folinic acid infusion therapy has recently become a popular regimen for various cancers. It is necessary that both oncologists and neurologists be fully aware of this unusual complication.