In vivo effects of the DNA hypomethylating agent 5-aza-2?-deoxycytidine (DAC) upon the epigenome of acute myeloid leukemia (AML) patients are scarcely studied in primary blasts. Here, we sequentially assessed DNA methylation and transcriptome changes in myeloblasts of DAC-treated AML patients. DNA methylation changes were detected in all patients already after the first series of infusion (median: 6 days). LINE1 analysis indicated global DNA methylation decrease in 7/8 patients. Gene-specific hypomethylating effects were not directly linked to mRNA expression changes. In conclusion, complex DAC-induced DNA methylation changes occur very early and imply mechanisms distinct from non-hypomethylating cytosine analogs.