Between 1996 and 2000, 90 newly diagnosed adult patients with T-acute lymphoblastic leukemia (T-ALL) were registered in the Gruppo Italiano Malattie Ematologiche dell'Adulto (GIMEMA) Leucemia Acuta Limfoide (LAL) 0496 protocol. Cases were centrally processed for morphology, immunophenotype, cytogenetics, molecular biology, and multidrug resistance (MDR). Twenty-two patients were females and 68 were males. Four percent of cases were pro-T, 47% pre-T, 39% cortical T, and 10% mature T-ALL. Fifty-six percent of patients with pro-T + pre-T-ALL achieved complete remission (CR) compared with 91% for cortical + mature cases (P = .002). CD34 expression was associated with a significantly lower CR rate: 54% versus 84% (P = .009). Thirty-one (36.5%) of 85 patients had an abnormal karyotype, the most common abnormality (15%) being a partial del(6q). The cytogenetic profile did not impact on CR achievement. MDR1 function, present in 26% of cases, correlated significantly with CR achievement (P = .004). A highly significant (P = .001) difference in CR rate was observed between patients who did not express the CD13/CD33/CD34 antigens and were MDR functionally negative (96%) compared with patients positive for at least one of these markers (57%). Multivariate analysis showed an impact on CR achievement for CD33 expression and MDR1 function. An extensive biologic workup of adult T-ALL cases at presentation is recommended in order to design tailored therapeutic strategies aimed at improving CR rates.