The present study was undertaken to investigate the regulation of P12CDK2AP1 by miRNAs. A conserved target site for miR-21 within the CDK2AP1-3′-UTR at nt 349–370 was predicted by bioinformatics software and an inverse correlation of miR-21 and CDK2AP1 protein was observed. Highly specific amplification and quantification of miR-21 was achieved using real-time RT-PCR. Transfection of HaCaT cells with pre-miR-21 significantly suppressed a luciferase reporter including the CDK2AP1-3′-UTR, whereas transfection of Tca8113 with anti-miR-21 increased activity of this reporter. This was abolished when a construct mutated at the miR-21/nt 349–370 target site was used instead. Anti-miR-21-transfected Tca8113 cells showed an increase of CDK2AP1 protein and reduced proliferation and invasion. Resected primary tumors and tumor-free surgical margins of 18 patients with head and neck squamous cell carcinomas demonstrated an inverse correlation between miR-21 and P12CDK2AP1. This study shows that P12CDK2AP1 is downregulated by miR-21 and that miR-21 promotes proliferation and invasion in cultured cells. J. Cell. Biochem. 112: 872–880, 2011. © 2010 Wiley-Liss, Inc.