The Runx2 (CBFA1/AML3/PEBP2alphaA) transcription factor promotes lineage commitment and differentiation by activating bone phenotypic genes in postproliferative osteoblasts. However, the presence of Runx2 in actively dividing osteoprogenitor cells suggests that the protein may also participate in control of osteoblast growth. Here, we show that Runx2 is stringently regulated with respect to cell cycle entry and exit in osteoblasts. We addressed directly the contribution of Runx2 to bone cell proliferation using calvarial osteoblasts from wild-type and Runx2-deficient mice (i.e., Runx2(-/-) and Runx2(DeltaC/DeltaC)). Runx2(DeltaC/DeltaC) mice express a protein lacking the Runx2 COOH terminus, which integrates several cell proliferation-related signaling pathways (e.g., Smad, Yes/Src, mitogen-activated protein kinase, and retinoblastoma protein). Calvarial cells but not embryonic fibroblasts from Runx2(-/-) or Runx2(DeltaC/DeltaC) mutant mice exhibit increased cell growth rates as reflected by elevations of DNA synthesis and G(1)-S phase markers (e.g., cyclin E). Reintroduction of Runx2 into Runx2(-/-) calvarial cells by adenoviral delivery restores stringent cell growth control. Thus, Runx2 regulates normal osteoblast proliferation, and the COOH-terminal region is required for this biological function. We propose that Runx2 promotes osteoblast maturation at a key developmental transition by supporting exit from the cell cycle and activating genes that facilitate bone cell phenotype development.