We report the first large series (n=596) of pediatric acute myeloid leukemia (AML) focusing on modal numbers (MN) from the population-based NOPHO-AML trials. Abnormal karyotypes were present in 452 cases (76%) and numerical aberrations were present in 40% (n=237) of all pediatric AML. Among patients with an abnormal karyotype, the MN 46 was most common (n=251; 56%) of which 36 (8%) were pseudodiploid with numerical aberrations, followed by MN 47 (n=80; 18%) and MN 43-45 (n=48; 8%). No cases had MN less than 43. Hyperdiploid AML with MN 48-65 comprised 11% of all cases and was associated with early onset (median age 2 years), female sex (57%), and a dominance of acute megakaryoblastic leukemia (AMKL) (29%). Hypodiploidy constituted 8% of all AML and was associated with older age (median age 9 years), male predominance (60%), FAB M2 (56%), and t(8;21)(q22;q22) (56%) with loss of sex chromosomes. Inferior outcome was observed for hypodiploid cases (5-year event-free survival 40% and 5-year overall survival 40%) but did not reach statistical significance. Chromosomes were gained in a nonrandom pattern, where chromosomes 8, 21, 19, and 6 were the most commonly gained. In conclusion, based on MNs, two cytogenetic subgroups with characteristic clinical features are described; hypodiploidy found in 8% and associated with high median age, male sex, t(8;21)(q22;q22), and FAB M2 and possibly associated with inferior outcome (P=0.13), and hyperdiploidy with MN 48-65 in 11% associated with early onset, female sex, and AMKL.