The multi-hit hypothesis for paediatric leukemogenesis states that an initial genetic hit (often occurring prenataly) must be followed by one or more hit(s) before a cell become leukeamic. Studies have demonstrated the presence of pre-leukaemic t(12;21)-positive cells at levels 10(-3) to 10(-4) in 1% of newborns (i.e. 100-fold their risk of t(12;21)-positive ALL), but only at levels of 10(-5) to 10(-6) in 0.5% adults. As the risk of developing ALL is inversely associated to the gestational age at birth, we investigated if this increased risk could be explained by an increase in prevalence and quantity of pre-leukaemic t(12;21)-positive children born prematurely. Using a sensitive qRT-PCR assay, we screened messenger RNA from fresh umbilical cord-blood samples from 256 premature children. In none of the neonates, t(12;21)-positive cells could be demonstrated. Therefore, no increase in the prevalence and magnitude of preleukaemic t(12;21)-positive cells compared to previously published data from mature children could be demonstrated. This indirectly supports the theory that prevalence and quantity of preleukaemic t(12;21)-positive cells peaks at term or early childhood and that exogenous factors are necessary to initiate their clearance.