Two children with osteosarcoma are presented in whom Wernicke encephalopathy with vomiting occurred during the chemotherapy. One of the children died with symptoms of toxic cardiomyopathy. Autopsy revealed Wernicke encephalopathy. The other child had similar symptoms (ocular signs, ataxia, somnolence). Parenteral thiamine had been given and after this therapy the child recovered from the encephalopathy. The authors emphasize the importance of the recognition of this neurological disorder occurring rarely in childhood: it can be cured with parenteral thiamine. Without thiamine treatment this condition is lethal.