WT1 gene expression has been proposed as a useful marker of minimal residual disease in leukaemia. Its utility in paediatric haematopoietic stem cell transplantation (HSCT) has not been studied. We studied the prognostic value of WT1 expression in peripheral blood prior to HSCT in 36 children with acute myeloid leukaemia (AML). Samples were obtained 2 weeks pre-transplant to determine the level of WT1 expression. WT1 expression was normalized using K562 cells as a control and a relative value of 0.5 was chosen as the cut-off point between high and low WT1 expression. The median level of pre-transplant WT1 expression in the 36 patients was 0.09 (range 0.0001-11.0), with 11 patients having WT1 >or= 0.5 and 25, WT1 < 0.5. After HSCT, 76% of patients with high pre-transplant WT1 expression relapsed, in contrast to 0% of the patients with low WT1 expression. Those with high WT1 expression had significantly lower 5-year event-free survival (EFS) (18%, 95% CI 0-40%) as compared to those with low WT1 expression (68%, 95% CI 50-86%, P = 0.007). Multivariate analysis showed that pre-transplant WT1 level is the only significant prognostic factor for the difference in EFS. Our finding suggests that elevated WT1 gene expression before HSCT in paediatric AML predicts relapse and poor long-term EFS. A larger prospective study is warranted to compare the value of high WT1 expression and other markers of minimal residue disease in predicting clinical outcomes after HSCT.