In an open-label, multicenter study carried out by Childrens Oncology Group, nelarabine was administered intravenously over 1 hour for 5 days to 151 patients ≤ 21 years of age, 149 of whom had relapsed or refractory T-cell acute lymphoblastic leukaemia (T-ALL) or T-cell lymphoblastic lymphoma (T-LBL).