In T-cell acute lymphoblastic leukemia (T-ALL), p53 gene mutations were found in 12 of 51 patients in first relapse (24%). In a retrospective study, bone marrow samples at diagnosis were obtained from 9 of the 12 relapsed patients with p53 mutation; only one patient was found to harbor a p53 mutation at diagnosis. No further p53 mutations were identified in 18 unpaired diagnosis T-ALL samples. This is the first report of a p53 mutation in T-ALL at diagnosis. p53 mutations in relapsed T-ALL were clinically relevant. Patients with p53 mutations experience a shorter duration of survival than those patients without p53 mutations. Additionally, patients with p53 mutations were significantly less likely to have achieved a complete second remission from reinduction therapy than those patients without p53 mutations and experience a shorter duration of survival from relapse even when a second reinduction is obtained. Though primarily identified only at relapse, p53 mutations were also associated with a decreased duration of first remission and overall decrease in survival from diagnosis. Patients with p53 mutations had a 3.8-fold increase in risk of death than those patients without p53 mutations. These findings suggest that p53 mutation is associated with poor clinical outcome that is characterized by (1) a shortened duration of survival after first relapse; (2) a reduced response to reinduction therapy; (3) a shortened duration of first remission; and, hence, (4) an overall decreased duration of survival and increased risk of death.