OBJECTIVE To study the expression of zinc finger protein X-linked (ZFX) in bone marrow mononuclear cells (BMMCs) of children with B lineage acute lymphoblastic leukemia (B-ALL) and its relationship with prognosis. METHODS The expression of ZFX in human leukemia cell lines (REH, HL-60, NB(4) and K562) was measured by Western blot. ZFX gene was cloned by PCR from one patient and DNA sequencing technology was used to confirm it. Real-time PCR was used for detecting ZFX mRNA expression in the BMMCs of 82 children with newly-diagnosed B-ALL, 24 children with complete remission (CR) after induction therapy and 64 control children (fracture or congenital heart disease patients). According to the presence of bone marrow or central nervous system relapse during a follow-up of 3 years, the patients were identified as having a good or poor prognosis. Their ZFX mRNA levels in BMMCs at diagnosis were compared. RESULTS ZFX protein was expressed in human leukemia cell lines REH, HL-60, NB(4) and K562. ZFX mRNA expression was significantly higher in the newly-diagnosed ALL group than in the control group (P < 0.01). ZFX mRNA expression in the ALL CR group was significantly reduced compared with the newly-diagnosed ALL group (P < 0.01). Children with a poor prognosis had significantly higher ZFX mRNA levels at diagnosis than those with a good prognosis (P < 0.05). CONCLUSIONS ZFX is over-expressed in children with B-ALL and its levels are higher in those with a poor prognosis than those with a good prognosis, which suggests that ZFX is important in the prognosis evaluation of B-ALL.