Abstract: The cell cycle distribution of bone marrow and/or blood samples from 86 patients with leukemias and myelodysplastic syndromes and 28 healthy controls were detected and analyzed with DNA-flow cytometry. The fraction of proliferative phase cells （S + G₂M） of the marrow in the majoraty of patients with ALL, ANLL, CML and CLL was Lower than in the normal controls （P＜0.001 or 0.05）, and the cell proliferation in acute leukemia individuals showed considerable variation.In acute leukemia, the percentage of S + G₂M in the marrow was higher than in the blood. No relationship was observed between S + G₂M% and morphologically identifiable blast cells in the original marrow and blood samples. The cell cycle dis tribution of marrow in myelodysplastic syndromes was related to the clinical categories and prognosis. The mean percentage of S + G2M in RA and RAS was close to the normals but that in RAEB and RAEB-T was similar to that in acute leukemia.