The biology and outcome of adult T-cell acute lymphoblastic leukemia are poorly understood. We present here the clinical and biologic features of 356 patients treated uniformly on the prospective trial (UKALL XII/ECOG 2993) with the aim of describing the outcome and identifying prognostic factors. Complete remission was obtained in 94% of patients, and 48% survived 5 years. Positivity of blasts for CD1a and lack of expression of CD13 were associated with better survival (P = .01 and < .001, respectively). NOTCH1 and CDKN2A mutations were seen in 61% and 42% of those tested. Complex cytogenetic abnormalities were associated with poorer survival (19% vs 51% at 5 years, P = .006). Central nervous system involvement at diagnosis did not affect survival (47% vs 48%, P = not significant). For 99 patients randomized between autograft and chemotherapy, 5-year survival was 51% in each arm. Patients with a matched sibling donor had superior 5-year survival to those without donors (61% vs 46%, χ², P = .02); this was the result of less relapse (25% vs 51% at 5 years, P < .001). Only 8 of 123 relapsed patients survive. This study provides a baseline for trials of new drugs, such as nelarabine, and may allow risk-adapted therapy in patients with poor-prognosis T-cell ALL.