Protein kinase C (PKC) plays an important role in the regulation of glioma growth; however, the identity of the specific isoform and mechanism by which PKC fulfills this function remain unknown. In this study, we demonstrate that PKC activation in glioma cells increased their progression through the cell cycle. Of the six PKC isoforms that were present in glioma cells, PKC α was both necessary and sufficient to promote cell cycle progression when stimulated with phorbol 12-myristate 13-acetate. Also, decreased PKC α expression resulted in a marked decrease in cell proliferation. The only cell cycle-regulatory molecule whose expression was rapidly altered and increased by PKC α activity was the cyclin-cyclin-dependent kinase (CDK) inhibitor p21^Waf1/Cip1. Coimmunoprecipitation studies revealed that p21^Waf1/Cip1 upregulation was accompanied by an incorporation of p21^Waf1/Cip1 into various cyclin-CDK complexes and that the kinase activity of these complexes was increased, thus resulting in cell cycle progression. Furthermore, depletion of p21^Waf1/Cip1 by antisense strategy attenuated the PKC-induced cell cycle progression. These results suggest that PKC α activity controls glioma cell cycle progression through the upregulation of p21^Waf1/Cip1, which facilitates active cyclin-CDK complex formation.