We observed leukoencephalopathy in 1 patient, and progressive dementia in another, during the administration of 5-fluorouracil (5-FU) and levamisole. A retrospective search, among 80 other patients with resected colorectal cancer receiving 5-FU and levamisole as adjuvant therapy, 166 resected malignant melanoma patients receiving adjuvant levamisole, and 254 advanced colorectal cancer patients receiving 5-FU often combined with leucovorin, for other cases of encephalopathy was negative. The frequency of this neurotoxicity is low (about 2% of patients receiving 5-FU and levamisole), but it appears specific for this combination of drugs. The lack of complete reversibility on stopping the drugs is worrisome, as this therapy is used to improve the curability of resected colon cancer.