Recurrent, prognostically significant chromosomal abnormalities occur in approximately 75% of paediatric acute lymphoblastic leukaemia (ALL), but only infrequently in children with Down syndrome (DS) and ALL. Recently, novel somatic activating mutations in the gene Janus kinase 2 (JAK2) were reported in 18% of DS ALL. Here we report identification and clinical correlates of JAK2 mutations in an independent cohort. JAK2 activating mutations occurred in 10/53 DS ALL cases (18·9%). Mutations were overrepresented in males (P < 0·03), occurred once in association with high hyperdiploidy and were not significantly correlated with age, initial white blood count, or event-free survival. Our results confirm the significance of JAK-STAT pathway activation in DS ALL. © 2008 Blackwell Publishing Ltd.