The messenger RNA of the intronless CEBPA gene is translated into distinct protein isoforms through the usage of consecutive translation initiation sites. These translational isoforms have distinct functions in the regulation of differentiation and proliferation due to the presence of different N-terminal sequences. Here, we describe the function of an N-terminally extended protein isoform of CCAAT enhancer-binding protein a (C/EBPa) that is translated from an alternative non-AUG initiation codon. We show that a basic amino-acid motif within its N-terminus is required for nucleolar retention and for interaction with nucleophosmin (NPM). In the nucleoli, extended-C/EBPa occupies the ribosomal DNA (rDNA) promoter and associates with the Pol I-specific factors upstream-binding factor 1 (UBF-1) and SL1 to stimulate rRNA synthesis. Furthermore, during differentiation of HL-60 cells, endogenous expression of extended-C/EBPa is lost concomitantly with nucleolar C/EBPa immunostaining probably reflecting the reduced requirement for ribosome biogenesis in differentiated cells. Finally, overexpression of extended-C/EBPa induces an increase in cell size. Altogether, our results suggest that control of rRNA synthesis is a novel function of C/EBPa adding to its role as key regulator of cell growth and proliferation.