In this study we aimed at investigating the pathways of recognition of acute lymphoblastic leukemia blasts by natural killer cells and at verifying whether differences in natural killer cell activating receptor ligands expression among groups defined by age of patients or presence of cytogenetic/molecular aberrations correlate with the susceptibility to recognition and killing. We analyzed 103 newly diagnosed acute lymphoblastic leukemia patients (46 adults and 57 children). Pediatric blasts showed a significantly higher expression of Nec-2 (p=.03), ULBP-1 (p=.01) and ULBP-3 (p=.04) compared to adult cells. The differential expression of these ligands between adults and children was confined to B-lineage acute lymphoblastic leukemia with no known molecular alterations. Within molecularly defined subgroups of patients, a high surface expression of NKG2D and DNAM1 ligands was found on BCR-ABL+ blasts, regardless of patient age. Accordingly, BCR-ABL+ blasts proved to be significantly more susceptible to natural killer-dependent lysis than B-lineage blasts without molecular aberrations (p=.03). Cytotoxic tests performed in the presence of neutralizing antibodies indicated a pathway of acute lymphoblastic leukemia cell recognition in the setting of the Nec-2/DNAM-1 interaction. These data provide a biologic explanation to the different role played by alloreactive natural killer cells in pediatric versus adult acute lymphoblastic leukemia and suggest that new natural killer-based strategies targeting specific subgroups of patients, particularly those BCR-ABL+, are worthy of being further pursued.