We report a case of a 4-year-old girl with diagnosed proB acute lymphoblastic leukaemia with co-expression CD33 antigen, treated according to Acute Lymphoblastic Leukaemia Intercontinental - Berlin Frankfurt Münster 2002 (ALL-IC BFM 2002) protocol for standard risk group. Haematological remission was obtained on day 33 of induction treatment (on time). During induction and consolidation therapy there were no early serious adverse effects. The late isolated bone marrow relapse of acute myeloid leukaemia, type 7 was noted in our patient. We recognized this case as a lineage switch acute lymphoblastic leukaemia to acute myeloid leukaemia. In spite of Ida Flag regimen and following Acute Myeloid Leukaemia - Berlin Frankfurt Münster 2004 (AML-BFM 2004) protocol were administered, the clinical and haematological remission was not achieved and the patient died because of disease progression (circulatory and respiratory insufficiency).