We herein report an innovative antisense approach based on Peptide Nucleic Acids (PNAs) to down-modulate CD5 expression levels in chronic lymphocytic leukemia (CLL). Using bioinformatics tools, we selected a 12-mer tract of the CD5 mRNA as the molecular target and synthesized the complementary and control PNA strands bearing a serine phosphate dipeptide tail to enhance their water solubility and bioavailability. The specific recognition of the 12-mer DNA strand, corresponding to the target mRNA sequence by the complementary PNA strand, was confirmed by non-denaturing polyacrylamide gel electrophoresis, thermal difference spectroscopy, circular dichroism (CD), and CD melting studies. Cytofluorimetric assays and real-time PCR analysis demonstrated the downregulation of CD5 expression due to incubation with the anti-CD5 PNA at RNA and protein levels in Jurkat cell line and peripheral blood mononuclear cells from B-CLL patients. Interestingly, we also observed that transfection with the anti-CD5 PNA increases apoptotic response induced by fludarabine in B-CLL cells. The herein reported results suggest that PNAs could represent a potential candidate for the development of antisense therapeutic agents in CLL.