New plant cells arise at the meristems, where they divide a few times before they leave the cell-cycle program and start to differentiate. Here we show that the E2Fa-DPa transcription factor of Arabidopsis thaliana is a key regulator determining the proliferative status of plant cells. Ectopic expression of E2Fa induced sustained cell proliferation in normally differentiated cotyledon and hypocotyl cells. The phenotype was enhanced strongly by the co-expression of E2Fa with its dimerization partner, DPa. In endoreduplicating cells, E2Fa--DPa also caused extra DNA replication that was correlated with transcriptional induction of S phase genes. Because E2Fa--DPa transgenic plants arrested early in development, we argue that controlled exit of the cell cycle is a prerequisite for normal plant development.