CDK4/cyclin D has a key role in regulating progression through late G1 into S phase of the cell cycle. CDK4/cyclin D complexes then persist through the latter phases of the cell cycle, although little is known about their potential roles. We have developed small molecule inhibitors which are highly selective for CDK4 and have used these to define a role for CDK4/cyclin D in G2 phase. Addition of the CDK4 inhibitor or siRNA knockdown of cyclin D3, the cyclin D partner, delayed progression through G2 phase and mitosis. The G2 phase delay was independent of ATM/ATR and p38MAPK, but associated with elevated Wee1. The mitotic delay was due to failure of chromosomes to migrate to the metaphase plate. However, cells eventually exited mitosis, with a resultant increase in cells with multiple or micronuclei. Inhibiting CDK4 delayed the expression of the chromosomal passenger proteins survivin and borealin, although this was unlikely to account for the mitotic phenotype. These data provide evidence for a novel function for CDK4/cyclin D3 activity in S and G2 phase that is critical for G2/M progression and the fidelity of mitosis.