A boy with acute lymphoblastic leukemia (ALL) who underwent lineage switch at relapse is reported. The second leukemia was myeloid in nature (acute myeloid leukemia, AML), characterized by predominantly My 9 positive blasts at first and at second relapse. Cytogenetic studies at second relapse revealed the translocation (9;11) (p21;q23) in all examined blasts. This is typical for myelomonocytic leukemia. The nature of the relapse and the occurrence of t(9;11) translocations in acute leukemia are discussed.