We describe the rare finding of a 33-month-old child neonatally diagnosed with Down syndrome, who presented with pre-B acute lymphoblastic leukemia (ALL) with a pretreatment bone marrow karyotype in which a low hypodiploid cell line (38 chromosomes) was identified in 17/19 cells studied. The abnormal cell line retained the extra constitutional chromosome 21. Hypodiploidy (loss of one or more chromosomes) is seen in approximately 5% of all childhood pre-B ALL cases and in approximately 2.2% cases of individuals with a constitutional trisomy 21. Low hypodiploidy, associated with a high risk of relapse, is rare in pediatric ALL cases in the general population, and, to our knowledge, is previously unreported in patients with trisomy 21.