['Expression of CD56 has been associated with poor prognosis in acute myeloid leukemia and aggressive lymphoma.', 'We analyzed the impact of CD56 expression in a cohort of 452 newly diagnosed adult T-cell acute lymphoblastic leukemia (T-ALL) patients; clinical data were available for 306 patients. Treatment was according to the GMALL study protocols 06/99 and 07/03 stipulating stratification into standard (thymic T-ALL) and high risk (pre- and mature T-ALL) groups.', 'CD56 expression was detected in 63/452 (13.9%) patients. CD56(+) T-ALL were predominantly of non-thymic (pre-T 35%, mature 41%) immunophenotypic subtypes, whereas 53% of the CD56(-) cases were thymic T-ALL (p=0.00002). CD13, CD33, CD34 and HLA-DR were significantly more frequently expressed. A clonal T-cell receptor rearrangement was detected in 22/23 CD56(+) ALL. No major clinical differences were observed at presentation. Treatment of CD56(+) ALL resulted in a lower rate of complete remissions (70% vs. 88%) (p=0.001) and a higher rate of resistant disease (21% vs. 8%) (p=0.004). CD56 expression had no significant influence on overall (48% vs. 59%) and disease free survival (67% vs. 57%) at three years.'] Is cD56 expression in T-cell acute lymphoblastic leukemia associated with non-thymic phenotype and resistance to induction therapy but no inferior survival after risk-adapted therapy?