Analgesic Effect in Children of Low-Dose Ketamine for Mucositis Treated by Patient-Controlled Analgesia With Morphine

Inclusion Criteria: * Painful mucositis * Child from 5 to 18 years. * Presenting mucitis induced by chemotherapy or a conditioning of graft hematopoïetic. * A pain not controlled by systematic paracetamol (10mg/kg/4h) or a the propacetamol (20mg/kg/4h). * Not presenting a neurological or psychological difficulty at the use of the PCA or a EVA. * First morphine amount going back to less 48h, and lass administration dating of more than 4h at the time of the beginning of the perfusion of Ketamine/placebo (EVA \> 30 after stop morphine). * Assent signed by the parents and each time possible by the child. Exclusion Criteria: * The child was already included in the study. * The age of psychomotor development is lower than 5 years * Presence of a depression or a major depressive episode according to definition DSM IV. * Hepatic or renal anomaly major (rank 3 or 4 of the scale of toxicity NCI CTC v2.0) * Bilirubin\>3N * Gamma WP\>3N * SGOT or SGPT \>5N * Creatinin\>3N