Cytogenetic studies on non-adherent cells from long-term bone marrow cultures (LTBMC) were done for 23 patients with myelodysplastic syndrome (MDS) and compared with the karyotypes detected during their clinical courses. Of 14 cases with normal karyotype before culture, abnormal karyotypes were detected first in LTBMC from seven. Novel abnormal karyotypes were observed after LTBMC in two of four cases which had had both normal and abnormal karyotypes before culture. Abnormal karyotypes were also newly detected in four of five cases with only abnormal karyotypes before culture. Among 13 cases in which abnormal karyotypes were observed during the cultures, three patients showed the same karyotypes 1-11 months later in their clinical courses. These findings suggest that our LTBMC might be useful for evaluating the prognosis and choice of treatment for MDS patients.