This study seeks to elucidate the molecular mechanisms by which the acute myeloid leukemia cell line, HL-60, is induced to differentiate into macrophage-like cells following exposure to phorbol 12-myristate 13-acetate (PMA). Changes in gene expression over the time course of PMA treatment were measured by DNA microarray analysis and select genes shown to be significantly up and down-regulated were validated by quantitative real-time PCR. Functional studies of transcription factor binding activities were performed by the electrophoretic mobility shift assay (EMSA). Novel findings include the rapid and sustained induction of mRNA levels for a group of CC chemokine genes (CCL2, CCL3, CCL3L1, CCL4, CCL4L1 and CCL4L2). These genes displayed 10 to 50-fold expression level increases that were observed within 3 hours of treatment and were maintained through 6 hours post-induction. Among the most highly induced early genes were others known to encode inflammatory response proteins (TNF, IL8, C3AR1). In addition, the transcription factor profile is dramatically altered early in the differentiation time course, with the predicted activation of AP-1 family members (JUNB, FOSL1) and the concomitant inhibition of NFKB (TNFAIP3, NFKBIA, NFKBID). c-Myc expression was significantly down regulated and remained low during the differentiation course. Early events in the genetic re-programming of HL-60 cells in response to the differentiation inducer PMA includes a massive up-regulation of chemokine genes and alteration of the transcription factor profile. The implication of these gene expression changes will be discussed in the context of differentiation therapies for acute myeloid leukemia.