Three cases of uncommon childhood hematologic disorders are reported. At presentation, one patient had refractory anemia with an excess of blasts (RAEB) with partial 7-monosomy and was reclassified into RAEB “in transformation” thereafter. Another case was diagnosed as acute myelogenous leukemia with complete 7-monosomy. The other case was diagnosed as RAEB “in transformation” without chromosome aberrations. The cytogenetic studies of the patients with 7-monosomy revealed abnormal karyotypes on bone marrow cells, but normal karyotypes on peripheral blood cells. Polymorphonuclear cells from the two patients with 7-monosomy revealed reduced mitochondrial malate dehydrogenase activity, but those from the patient with RAEB “in transformation” without chromosome aberrations did not. Cytoplasmic malate dehydrogenase activity, having been defined as located on chromosome 2, was within the normal range in those three patients. The decreased mitochondrial enzyme activity in the two patients with 7-monosomy would be a dosage effect of the chromosome aberration, but not caused by their hematologic disorders. The level of mitochondrial enzyme activity in the patients with 7-monosomy was reduced in polymorphonuclear cells, but not in mononuclear cells in peripheral blood. This fact would indicate that such chromosome evolution had involved myeloid cells only, but not lymphoid cells. Both enzymes from leukemic cells of four patients with active disease revealed much higher activities than controls, an expression of partially enhanced oxidative phosphorylation.