Abstract： Objective Repots of the clinical feature,immunophenotype and cytogenetic trait for children at age of less than 2 year old with newly diagnosed acute lymphoblastic leukemia(AEL).MethodsBetween 1996.1～2005.12 in Beijing Children's Hospital. children less than 2 year old with ALL were enrolled.Using retrospective methods,we collected and studied the clinical data,following up till to August 31.2006.Results There were 41 children with ALL at age of less than 2 year old.accounting for 3.9% of allin the same time,including 11 for the age less than 12 months old,21 with WBC more than 20×109/L 5 with center nervous system(CNS)leukemia.All cases are B-cell immunophenotype,22 with immunophenotypic co-expression of myeloid associated antigens,17 cytogenetic abnormalities and fusion gene positive,among them,7 involving ALL-1/MLL 11q23 gene rearrangements.Among 41 cases.28 cases were given combined intensive chemotherapy and achieved complete remission(CR)with induction therapy for 28 days,4 cases were relapsed from CR 9～34 months(medium 18 months).22 cases were follow-up continuously during 8 to 124 months(medium 39 months),continue complete remission(CCR)from 1 to 124 months(medium36.5 months).Conclusion Children with ALL at the age less than 2 year old display unique biological features as compared with older children.is associated with a hish leukocyte count at presentation and CNS involvement. The immunophenotype is usually B-lineage and is characterized by the co-expression of myeloid-associated antigens. and presence of involving ALL-1/MLL 11q23 gene rearrangements. Marrow relapse remains the primary mode of failure. Cytogenetic abnormalities involving ALL-1/MLL 11q23 gene rearrangements with ALL have a very poor outcome.