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).

Inclusion Criteria:

• ≤ 21 years of age
• Relapsed or refractory T-lineage acute lymphoblastic leukaemia (T-ALL) or lymphoblastic lymphoma (T-LBL)
• Prior treatment with at least two chemotherapy regimens
• Selected for treatment with nelarabine

Exclusion Criteria:

• Known hypersensitivity to the active substance.
• Women of childbearing potential who are unable or unwilling to use adequate contraceptive measures
• Males with partners of child bearing potential who are not willing to use condoms or abstinence
• Patients with persistent neurological toxicity (CTC grade > = grade 2)
• Adolescents (aged 16 years and over) and adults for whom the physician prescribes the 1500mg/m2 dose of nelarabine.

Male or Female 21 years or younger with Relapsed or refractory T-lineage Acute Lymphoblastic Leukaemia (T-ALL) or lymphoblastic lymphoma (T-LBL)