Myelodysplastic syndrome (MDS) has been thought to be an identifiable early stage in multistep leukemogenesis. Considerable numbers of patients with MDS eventually develop acute myelogenous leukemia (AML), which is very much more difficult to manage than typical denovo AML. There are several differences in both the clinical and biological behavior of AML with and without prior MDS. We have established an unique long-term bone marrow culture (LTBMC) system which allows abnormal cells to grow in preference to normal cells, based on the method originally described by Dexter et al. Leukemic transformations occur more frequently in MDS patients with abnormal karyotypes, and particularly those with multiple abnormalities. New cytogenetic abnormalities were occasionally observed at the time of transition to AML. We have applied this LTBMC system for cytogenetic and molecular studies on the leukemic transformation from MDS. Among the 32 patients with MDS studied thus far, novel abnormal karyotypes were detected during the LTBMCs in 15 patients. Furthermore, 6 out of 23 novel karyotypes detected during the in-vitro cultures emerged in vivo, one to 11 months later in 4 patients. In addition, point mutation of the N-ras proto-oncogene was observed in 3 of 18 cases. The signal of the dot-blot hybridization was increased in one examined case after 2 weeks in culture. Thus, this LTBMC system may provide some promising information with respect to understanding the multistep process from the preleukemic stage to the development of overt leukemia as well as its prognostic relevance.