One-hundred thirty-nine patients with acute lymphoblastic leukemia (ALL) and hypodiploidy (fewer than 45 chromosomes) were collected from 10 different national ALL study groups and single institutions. Patients were stratified by modal chromosome number into 4 groups: 24 to 29 (N = 46); 33 to 39 (N = 26); 40 to 43 (N = 13); and 44 (N = 54) chromosomes. Nine patients were Philadelphia chromosome (Ph) positive (4 cases: 44 chromosomes; 5 cases: 40-43 chromosomes) and were not considered further. Event-free survival (EFS) and overall survival (OS) of the remaining 130 patients were 38.5% +/- 4.4% and 49.8% +/- 4.2% at 8 years, respectively. There were no significant differences in outcome between patients with 24 to 29, 33 to 39, or 40 to 43 chromosomes. Compared with patients with fewer than 44 chromosomes, patients with 44 chromosomes had a significantly better EFS (P = .01; 8-year estimate, 52.2% vs 30.1%) and OS (P = .017; 69% vs 37.5%). For patients with 44 chromosomes, monosomy 7, the presence of a dicentric chromosome, or both predicted a worse EFS but similar OS. Doubling of the hypodiploid clone occurred in 32 patients (24-29 chromosomes [n = 25] and 33-39 chromosomes [n = 7]) and had no prognostic implication. Children and adolescents with ALL and hypodiploidy with fewer than 44 chromosomes have a poor outcome despite contemporary therapy.