BACKGROUND:: Several large biobanks comprising umbilical cord blood samples have been established allowing efforts to characterize the prevalence and risk factors for preleukemic cell clones in healthy newborns. This study explores the feasibility of demonstrating translocation ETV6-RUNX1 transcripts by reverse transcription polymerase chain reaction in newborns enrolled in a large Danish National Birth Cohort. CONCLUSIONS:: The study emphasizes the necessity of either storing viable cord blood cells or preparation of the RNA within 1 to 2 days of birth, in large scale studies of the natural history of childhood acute lymphoblastic leukemia. Finally, the estimated frequency of translocation ETV6-RUNX1-positive cells was below 10.