A plethora of extracellular stimuli regulate growth, survival, and differentiation responses through activation of the MEK-ERK MAPK signaling module. Using CD34+ hematopoietic progenitor cells, we describe a novel role for the MEK-ERK signaling module in the regulation of proliferation, survival, and cytokine production during neutrophil differentiation. Addition of the specific MEK1/2 inhibitor U0126 resulted in decreased proliferation of neutrophil progenitors. Conversely, transient activation of a conditionally active MEK1 mutant resulted in the expansion of progenitor cells, which thereafter differentiated normally into mature neutrophils. In contrast, chronic MEK1 activation was found to induce cell death of CD34+ neutrophil progenitors. Microarray analysis of CD34+ progenitor cells showed that activation of MEK1 resulted in changes in expression of a variety of cell-cycle modulating genes. Furthermore, conditional activation of MEK1 resulted in a dramatic increase in the expression of mRNA transcripts encoding a large number of hematopoietic cytokines, chemokines, and growth factors. These findings identify a novel role for MEK-ERK signaling in regulating the balance between proliferation and apoptosis during neutrophil differentiation, and they suggest the need for tight control of MEK-ERK activation to prevent the development of bone marrow failure.