Inclusion Criteria:

* T-ALL patients must be enrolled on AALL08B1 prior to treatment and enrollment on AALL0434
* Patients must have newly diagnosed T-ALL or T-lineage lymphoblastic lymphoma (T-NHL) stage II-IV; B-lineage lymphoblastic lymphoma will not be eligible for this study; a diagnosis of T-ALL is established when leukemic blasts lack myeloperoxidase or evidence of B-lineage derivation (cluster of differentiation \[CD\]19/CD22/CD20), and express either surface or cytoplasmic CD3 or two or more of the antigens CD8, CD7, CD5, CD4, CD2 or CD1a; if surface CD3 is expressed on all leukemic cells, additional markers of immaturity, including transmission disequilibrium test (TdT), CD34 or CD99 will be assessed for expression; cases with uncertain expression will receive additional review within the appropriate Children's Oncology Group (COG) reference laboratory
* T-NHL PATIENTS:

* For T-NHL patients with tissue available for flow cytometry, the criterion for diagnosis should be analogous to T-ALL; for tissue processed by other means (i.e. paraffin blocks), the methodology and criteria for immunophenotypic analysis to establish the diagnosis of T-NHL defined by the submitting institution will be accepted
* Prior therapy restrictions

* Patients shall have had no prior cytotoxic chemotherapy with the exception of steroids and/or IT cytarabine
* IT chemotherapy with cytarabine is allowed prior to registration for patient convenience; this is usually done at the time of the diagnostic bone marrow or venous line placement to avoid a second lumbar puncture; (Note: the CNS status must be determined based on a sample obtained prior to administration of any systemic or intrathecal chemotherapy, except for steroid pretreatment); systemic chemotherapy must begin within 72 hours of this IT therapy
* Patients diagnosed as having T-NHL or T-ALL with respiratory distress or hyperleukocytosis may require steroids prior to the initiation of additional systemic therapy; they are eligible for AALL0434 and will be stratified, based on the initial complete blood count (CBC); steroid pretreatment may alter the risk group assessment; if the T-ALL patient's clinical status precludes a lumbar puncture within 48 hours of the initiation of steroid therapy, T-ALL patients CANNOT be classified as low risk and will be Intermediate or high risk based on the results of the day 29 marrow as above; patients with T-NHL who receive steroid pre-treatment will be classified as high risk; the dose and duration of previous steroid therapy should be carefully documented
* For the management of airway compromise, patients who have received emergent chest irradiation up to 600 cGy will be eligible for this study
* Patients with a prior seizure disorder requiring anti-convulsant therapy are not eligible to receive nelarabine; in addition, patients with pre-existing grade 2 (or greater) peripheral neurotoxicity, as determined prior to Induction treatment by the treating physician or a neurologist, are not eligible to receive nelarabine; these restrictions in eligibility are designed to prevent excessive nelarabine-induced central and peripheral neurotoxicity in at-risk patients; for the purposes of this study, this includes any patient that has received anticonvulsant therapy to prevent/treat seizures in the prior two years

Exclusion Criteria:

* Pregnant or lactating females are ineligible
* Patients with Down syndrome are ineligible to enroll onto this study
* For T-NHL patients the following additional exclusion criteria apply:

* B-precursor lymphoblastic lymphoma
* Morphologically unclassifiable lymphoma
* Absence of both B-cell and T-cell phenotype markers in a case submitted as lymphoblastic lymphoma
* CNS3-positive or testicular involvement