Acute lymphoblastic leukemia (ALL) of childhood is frequently characterized by a hyperdiploid karyotype. Typically, most of the affected chromosomes in the abnormal clone are present in three copies. We have studied two patients with hyperdiploid ALL whose leukemic cells were atypical in that all or most of the chromosomes were present in either two or four copies, raising a suspicion that the observed karyotype arose through duplication of chromosomes in a precursor cell with a near-haploid chromosome number. Analysis of restriction fragment length polymorphisms confirmed that both cases arose from a near-haploid cell; all informative disomic chromosomes tested had loss of heterozygosity. Furthermore, the hyperdiploid karyotypes did not arise via a perfect haploid cell with exactly 23 chromosomes, because tetrasomic chromosomes remained heterozygous. These two patients probably are classified best as near-haploid cases, which often are observed to have a co-existing hyperdiploid clone with a duplicated chromosome set. The distinction between typical hyperdiploidy and hyperdiploidy arising via a near-haploid cell may be clinically important, because the prognosis for patients with a hyperdiploid karyotype is favorable in comparison to that of patients with a near-haploid karyotype.