Background: The Myelodysplastic Syndromes (MDS) are disorders involving clonal proliferative activity in the bone marrow that can lead to marrow failure and acute leukemia. This study was undertaken to evaluate the relationships between clinical factors, type of MDS, DNA ploidy, and Ki-67 expression. Design: Air-dried bone marrow smears from 27 patients were Feulgen stained for DNA and analyzed using the CAS 200 image analyzer. Ki-67 expression was also examined by image analysis in 25 of these cases in bone marrow core biopsy specimens using the monoclonal antibody MIB-1. Age, sex, bone marrow cellularity, and MDS grade of each patient were also recorded. Percent S-phase was assessed only for the diploid samples. Results: There were 16 diploid and 11 aneuploid cases on analysis of Feulgen-stained bone marrow aspirate smears. The percentage of MIB-1+ cells ranged from 6.4%-61.7% (mean - 37.7 ± 6.4%). Among the 16 diploid cases, 18.2 ± 5.7% of the cells were in S-phase. High grade MDS (RAEB-T, RAEB, CMML) was associated with younger age and male sex (p = 0.03), lower percentage of cells in S-phase (p = 0.04), greater bone marrow cellularity (p = 0.005), and greater MIB-1 expression p = 0.04). With increasing age, there were more females (p = 0.03), more low grade MDS (RA, RARS), and a lower percentage of cells in S-Phase (p = 0.04). Female patients tended to be older, have less cellular bone marrows (p = 0.003), less MIB-1 expression (p = 0.03), low grade MDS (p = 0.02), and increased percentage of cells in S phase (p = 0.008). Conclusion: Ki-67 expression, percent cells in S-phase, clinical parameters and subtype of MDS tend to distinguish two separate groups of MDS patients, and may explain in part the difference in biologic behaviour between high and low grade MDS.