Leukemia diagnosed during the first year of life is a rare but highly malignant disease of hematopoietic system. In contrast to leukemias diagnosed after the first year of life, infant leukemia is characterized by distinct biological and genetic features, which influence the clinical course and disease outcome. Approximately 60% of acute leukemias in infants are associated with aberrations of chromosome 11q23 with the rearrangement of the MLL gene. Despite major progress in treatment of childhood leukemias, prognosis of infant leukemia (particularly in acute lymphoblastic leukemia--ALL) remains poor. The most unfavorable prognostic factors in infant ALL are age at diagnosis below 6 months, CD10 negativity, myeloid antigen co-expression, presence of MLL gene rearrangements and in particular poor response to steroids. Long-term event free survival (EFS) in infant ALL ranges from 20 to 40%, depending on treatment protocol. Slightly better results are achieved in infant acute myeloid leukemia.