Cytogenetic studies of two patients with acute myeloblastic leukemia, classified as M1 and M2, showed a partial duplication of the distal part of the long arm of chromosome 12 (12q24) as the sole detectable chromosome abnormality. High white blood cell count was common to the two leukemic children. Fluorescence in situ hybridization analysis with the use of a whole-chromosome 12 painting probe and a telomeric probe confirmed the apparent absence of any translocation. The genes involved in the duplication could not be ascertained, but several candidate genes such as growth-factor receptors or genes encoding transcription factors localized to 12q22-q24 should be investigated in the molecular analysis of these duplications.