Objective:To investigate the prognostic value of cytoplasmic μ heavy chain(C-滋) in childhood immature B-cell acute lymphoblastic leukemia, in order to provide evidence for rational treatment protocol. Methods:From Jan 1, 2001 to March 31, 2005, 161 patients with immature B-cell acute lymphoblastic leukemia received ALL-XH-99 protocol treatment in our hospital. Baseline data were obtained for age, sex, peripheral white blood cell (WBC) count, immunophenotype, P170 and fusion gene. Absolute peripheral blood juvenile cell count of the eighth day of prednisone treatment, bone marrow smear of the eighteenth day of induction treatment, bone marrow minimal residual disease(MRD) after induction treatment, and risk classification were also assessed. Results:Kaplan-Meier analysis showed that the five-year event-free survival(EFS) rate of Cμ positive patients was 56.7 ± 0.88%, significantly lower than that of the Cμ negative patients(74.1 ± 0.05%,P < 0.01). χ2 analysis indicated that the prognostic difference was unrelated to biological characters such as sex, age, peripheral WBC count when first diagnosed, P170 level, myeloid marker (My), fusion gene(BCR/ABL or MLL/AF4), prednisone window trial and bone marrow smear of the nineteenth day of treatment. Cox regression risk analysis demonstrated that Cμ positive, WBC≥50×109/L before treatment, MRD positive and fusion gene positive were independent risk factors(P < 0.05). Conclusion:Cμ positive is an important risk factor of poor prognosis for childhood immture B-cell lymphoblastic leukemia, and it was independent of CD10 negative MLL gene rearrangements.