Nelarabine, an antimetabolite prodrug, is approved as monotherapy for children and adults with relapsed and refractory T-cell acute lymphoblastic leukemia and lymphoma (R/R T-ALL/LBL), although it is often used in combination regimens. We sought to understand differences in efficacy and toxicity when nelarabine is administered alone or in combination. We retrospectively analyzed 44 consecutive patients with R/R T-ALL/LBL; 29 of whom were treated with combination therapy, most with cyclophosphamide and etoposide (23, 79%) and 15 with monotherapy. The median age was 19 years (range, 2-69), including 18 children (<18 years). After a median of 1 (range, 1-3) cycle of treatment, 24 patients (55%) achieved complete remission, 62% (18/29) with combination therapy and 40% (6/15) with monotherapy (P = .21). Most responders (21, 88%) pursued allogeneic stem cell transplant (alloSCT). Overall survival (OS) was 12.8 months (95% confidence interval, 6.93-not reached) in the entire cohort and was higher in the combination therapy than in the monotherapy group (24-month OS, 53% vs 8%; P = .003). The rate of neurotoxicity was similar between groups (27% vs 17%; P = .46) and grade 3/4 anemia and thrombocytopenia were more frequent in the combination group (76% vs 20%; P < .001% and 66% vs 27%; P = .014, respectively). In a multivariate analysis, nelarabine combination therapy and alloSCT post nelarabine were associated with improved OS (hazard ratio, 0.41; P = .04 and hazard ratio, 0.25; P = .008, respectively). In conclusion, compared with monotherapy, nelarabine combination therapy was well tolerated and associated with improved survival in pediatric and adult patients with R/R T-ALL/LBL.