This international, multicentre, single arm, phase IV study will assess the safety and efficacy of nelarabine in children and young adults with relapsed or refractory T-lineage acute lymphoblastic leukaemia (T-ALL) or lymphoblastic lymphoma (T-LBL) whose disease has not responded to or has relapsed following treatment with at least two chemotherapy regimens. It is a post-authorisation safety study (PASS) conducted for the purpose of confirming the safety profile and the clinical benefit of nelarabine under licensed conditions of use. The study is observational, non-interventional, and will include approximately 40 children and young adults (up to 21 years of age).