In several large phase II trials, low-dose treatment with the azanucleoside 5-aza-2'-deoxycytidine (decitabine, DAC) resulted in complete hematologic and cytogenetic responses in 23 and 31% of MDS patients, respectively. The question of induction of chromosomal instability by this demethylating agent was addressed by serial karyotypic analyses. 53/122 DAC-treated patients had all normal metaphases at time of treatment start. In 46/53 patients, sequential cytogenetic analyses were performed. 9/46 patients (20%) acquired clonal chromosomal abnormalities during follow-up (4/9 transient). 8/9 abnormalities were gains or losses of entire chromosomes. The rate and pattern of cytogenetic evolution are thus not higher than in historical MDS cohorts not receiving specific treatment.