['Not all natural killer (NK) cells are equally cytotoxic against leukemia because of differences in receptor gene content and surface expression. We correlated NK cell genotype and phenotype at diagnosis of childhood acute lymphoblastic leukemia (ALL) with minimal residual disease (MRD) after induction chemotherapy.', 'The NK cells and leukemia blasts of 244 patients were analyzed at diagnosis by killer-cell immunoglobulin-like receptor (KIR) typing and immunophenotyping. The results were correlated statistically with postinduction MRD status.', 'The odds of being MRD positive in patients with KIR telomeric (Tel)-A/B genotype were 2.85 times the odds in those with Tel-A/A genotype (P = 0.035). MRD-positive patients were more likely to have KIR2DL5A (P = 0.006) and expressed less activating receptor NKp46 and FASL on their NK cells (P = 0.0074 and P = 0.029, respectively). The odds of being MRD positive increased by 2.01-fold for every percentage increase in NK cells expressing KIR2DL1 in the presence of HLA-C2 ligand (P = 0.034). The quantity of granzyme B inhibitor PI-9 in the leukemia blasts was greater in patients who were MRD positive (P = 0.038). Collectively, five NK cell-related factors (Tel-B-associated KIR2DL5A, NKp46, FASL, granzyme B, and PI-9) are strongly associated with MRD positivity at the end of induction with 100% sensitivity and 80% specificity.'] Do nK cell genotype and phenotype at diagnosis of acute lymphoblastic leukemia correlate with postinduction residual disease?